Principal Leadership and Teacher Professional Development in a Vietnamese High School for Gifted Students: Perspectives into Practice

Teacher professional development (TPD) is an important component of enhancing student learning and school education quality. Vietnamese Ministry of Education and Training is reforming education, including curricula, teaching methods, and textbooks. Many requirements have to been done to effectively implement the reform, and TPD is considered a significant contribution. Using data from an in-depth interview with the principal, teacher questionnaires, and school policy-related documents, this qualitative case-study research in a selected high school for gifted students in Central Vietnam aimed to find out how TPD had been implemented in response to education reforms in Vietnamese high schools. TPD was widely perceived and respected by the principal and teachers in this case study, and these positive perspectives were transferred into practice with numerous effectively employed TPD strategies at this high school for students’ enhanced learning and achievements.


Introduction
Teacher professional development (TPD) consists of processes and activities aimed at improving or enhancing teachers' teaching-related knowledge, teaching and educating skills, and teaching methods which will surely help them, as a result, develop suitable teaching programs to enhance student achievements and successes (Correnti, 2007;Hallinger et al., 2021;Newmann et al., 2000). High-quality TPD can provide teachers with subject knowledge and related skills that bring about a change in their beliefs, attitudes and actions which have ultimately positive influences on students' outcomes (Didion et al., 2019). TPD is considered to play an important role in students' learning improvements. Four major trends have been identified on TPD in the global literature over the past decades. First, TPD for in-service teachers is just as essential as pre-service teacher preparation at university (Lieberman & Pointer Mace, 2008). Secondly, more schoolbased learning activities have replaced traditional TPD events such as 'in-service seminars' and 'certificate/degree upgrading schemes' (Opfer & Pedder, 2011;Qian et al., 2017;Webster-Wright, 2009). Thirdly, TPD occurs in professional learning groups (Little, 2012;Wang, 2016). Finally, TPD is a critical component of long-term educational reforms (Fullan, 2011;Hallinger et al., 2021;Ping et al., 2018;Tran, Ha & Tran, 2021).
In developing nations, where the university-based pre-service teacher training quality and scope of need more renovations to meet the educational changes, having recently received increasing research attention regarding TPD (Hairon & Dimmock, 2012;Hallinger & Liu, 2016;Hallinger et al., 2017;Hallinger et al., 2021;Harris & Jones, 2019;Lai et al., 2016;Qian et al., 2017;Wang, 2016). The Vietnamese Communist Party and the Vietnamese government have emphasized the importance of principals' and teachers' capabilities to master new subject knowledge, related skills, and teaching methods during their service to ensure the successful implementation of education reforms (Hallinger et al., 2021;Ministry of Education and Training [MoET], 2013;Sun-Keung et al., 2016;Tran et al., 2018).
This research was carried out within these developments in a high school for gifted students in Ha Tinh † province in Central Vietnam where research contributions for international teacher professional learning knowledge have remained rather limited, to explore the TPD and principal leadership in that high school. The research question is "How is teachers' professional development for better teaching and learning implemented in Ha Tinh Provincial High School for Gifted Students in Vietnam?" This paper describes a high school case study in Ha Tinh province, Vietnam. Qualitative data were collected from the school principal's interview, questionnaires from the teachers, on-site observation of TPD at school and school-related document analysis. Thus, the study emphasizes the cultural components of principal's support, which are unique to Vietnam's political, educational and social context. The paper also aims to contribute to the international knowledge on TPD and to provide implications for educational stakeholders of different levels in Vietnam.

Literature Review
The research and practice on teacher professional learning have moved towards a multi-faceted and job-embedded trend for the past few decades (Hallinger et al., 2021;Qian et al., 2017). Thus, today teacher training is viewed on a continuum that begins during pre-service teacher training and continues throughout the teachers' teaching job (Little, 2012;Vescio et al., 2008;Webster-Wright, 2009). Studies on TPD has come to a conclusion that it is a continuing TPD process for teachers to achieve new subject knowledge, teaching skills, general knowledge and other supplementary skills (Ha & Murray, 2021;Hallinger et al., 2021;Lai et al., 2016;Opfer & Pedder, 2011;Ping et al., 2018;Tran, Ha & Tran, 2021;Tran, Le, Nguyen et al., 2021;Tran, Truong, et al., 2020). When successful, teacher development not only results in enhanced capacities for teaching and learning because teachers' enhanced professional development have positive impacts upon student improved learning and success and school achievements as well (Correnti, 2007;Newmann et al., 2000). Meissel et al. (2016) even state that, "Quality teaching makes a difference in student learning and the professional learning of teachers is a central factor in determining the quality of teaching" (p. 169).
Teachers' professional learning programs have been mapped out by several researchers (e.g., Borko, 2004;Opfer & Pedder, 2011). Within the school, activities such as seminars or peer observations of other teachers, and the activities outside the school, including workshops or degree upgrading programs, are two common forms of TPD. Professional learning may be individual-based, such as self-learning or online learning, or in a group-based or community-based environment, such as a teacher learning group or professional mutual observations Hallinger et al., 2021;Lai et al., 2016;Little, 2012;Opfer & Pedder, 2011;Ping et al., 2018;Prenger et al., 2017). Finally, TPD can be either obligatory or optional, and it is participated by teachers' voluntary participation in their activities or by the pressures externally resulting from the principal or senior educational authorities or MoET.
This new perspective on TPD has prompted researchers and teachers to find out ways that teachers can learn at work (Hallinger et al., 2021;Timperley, 2011). Teachers can develop their professional skills through formal frameworks, including professional learning programs, research groups, observations of other colleagues' lessons, mentorship, and coaching (Timperley, 2011;Tran, Truong, et al., 2020;Webster-Wright, 2009). When participating in jointly-shared assessment and informal conversations, teachers can learn in a more informal way (Little, 2012;Tran et al., 2018).

Methodology
This study used a case study research design (Patton, 2015;Yin, 2014) to investigate how the principal and his staff perceive the roles of TPD and how TPD strategies have been employed in reality in Ha Tinh Provincial High School for Gifted Students in Ha Tinh province in Central Vietnam. The descriptions of the sample, the data collection and analysis methods will be included in this part.

Sample
The purposeful sampling getting "information-rich cases whose study will illuminate the questions under study" (Patton, 2015, p. 169) is applied in this study. Small sample size is a factor of phenomenological research (Patton, 2015). The aim of the phenomenological study is to seek "thick description" of teacher professional learning at a high school for gifted students in Central Vietnam, rather than trying to claim generalization of a phenomenon. This high school has been identified to be considered most successful and famous one by Ha Tinh Provincial Department of Education and Training Service (PETS), based on the achievements of teachers' and students such as the number of good teachers' titles, the students' achievements and so on. As a result, the school has received the first grade labor medal awarded by the President of Vietnam and many other awards, Certificates of Merits by the Prime Minister, MoET, and President of Ha Tinh Provincial People's Committee.
Ha Tinh Provincial High School for Gifted Students has attracted talented high school students of Grade 10, Grade 11 and Grade 12 from all parts of the province. It was founded in 1991 with the aim of "discovering and nurturing gifted students at high school educational level in the province in order to train talented people for the province in particular and the country in general" (report of the school). It is located in Ha Tinh city, Ha Tinh province, Central Vietnam. Now, the School has 101 staff, including 88 teachers, one principal and three vice-principals, and other nine support staff. Amongst 92 teaching staff, two teachers hold a PhD degree, 56 teachers have a Masters' degree, other 15 teachers are doing a PhD or MA course.
There are thirty classes with 950 students at 10 th grade, 11 th grade, and 12 th grade. These classes are named, according to their major subjects, including specialized Maths, Physics, Chemistry, Literature, History, Geography, Biology, Informatics, French and English classes. Although the students study all subjects as do other students in normal high schools, they must focus on their major subjects. Every year, they have to take part in provincial and national tests for gifted students.
Students of the school have achieved great success in recent years, according to the school reports. The rate of the students passing 'kì thi tốt nghiệp trung học phổ thông' (the national high school education graduation examination) at the end of the 12 th grade is always 100 percent. Nearly 100 percent of the 12 th grade students pass the university entrance examination ‡ . After nearly 30 years, from 1992 to 2020, there have been 5697 high school graduates, 1030 grade 12 students who have passed the university entrance examination and 1342 students attaining the 'National Good Student (MoET) § ' title, of which 33 First Prize holders, 214 Second Prize holders, 553 Third Prize holders and 525 Encouragement Prize holders. The number of students achieving MoET National Good Student Title have helped the School rank amongst the top 5 best high schools in Vietnam.

Data Collection
The data was collected from the semi-structured interview with the principal, the teachers' questionnaires, and direct observations of TPD activities employed for teachers at Ha Tinh Provincial High School for Gifted Students. The semistructured interview in Vietnamese at his room for about 90 minutes was conducted to learn about the principal's perspectives on TPD and how his vision and perspectives had been transformed into productive TPD strategies for his teachers, as well as the essence of professional learning strategies for the teachers at the school (Patton, 2015).
Teachers' perspectives were gathered from their open-ended questionnaires focusing on their perceptions, viewpoints and motivations, as well as the professional development programs that they participated at their school. The 91 questionnaires handed out at the faculty meeting were all filled out and 82 questionnaires returned (90.1 percent response rate). Teacher professional development practices were studied for two and a half months, nearly an academic term, to obtain additional information and confirm what had been recorded in the interview and questionnaires. These observations provided multiple perspectives and allowed triangulation to be used to verify the credible validity of various perspectives as Patton (2015) recommended.

Data Analysis
The 'within-case analysis' (Patton, 2015;Yin, 2014) was used in this research. This process included developing comprehensive write-ups for Ha Tinh Provincial High School for Gifted Students. Analytical procedures began with data coding based on various data sources. We reanalyzed the data after finishing the case record, concentrating on identifying TPD practices used in Ha Tinh Provincial High School for Gifted students. To produce both lists of strategies in which they were used, open coding and axial coding were used (e.g., provided externally or school based, job-embedded or taught). The four primary steps of phenomenological analysis adopted from Patton (2015) were undertaken as follows: 1. Epoche: The case database was repeatedly read to achieve a sense of the whole, then two themes (external and school-based activities) were identified and categorized; 2. Phenomenological reduction: the case data was coded into free nodes; 3. Imaginative variation: the free nodes were re-examined to identify the "units of significance", and then coding into tree nodes via the two themes; 4. Synthesis of texture and structure: This process involved an integration of structural descriptions, providing a synthesis of the meanings and essences of the case. This paper was primarily reported on the data that emerged from step 4 of the data synthesis.

Perceptions of the significance of teachers' professional development
Thay Hoan recognized the close cause-effect links between teacher quality and student progress and achievements at his school. In the interview, he said, "không thầy đố mày làm nên" (you could not become successful without the supportive assistance of your teachers). Thay Hoan strongly emphasized: As for me, I believe that the quality of any teaching staff is critical to the quality of education, or, to put it another way, to the achievement, academic performance, and practice of students. While each student's actions are the most important factor, teachers have a significant and direct effect on the quality of each student. (Interview with Thay Hoan, henceforth THI) The principal's perspective was supported by all the teachers. When answering the question, 'What is the teacher's involvement in their students' success?', many positive words were used in the questionnaires, such as 'essential' 'major', 'extremely important', and 'very important'. Significantly, 75 out of 82 teachers used the word 'decisive' in their responses, and many teachers' questionnaires included the sentence "thầy giỏi ắt có trò hay" (excellent teachers will undoubtedly help students become excellent).
The province and the school management board have placed special attention on teacher quality because students' accomplishments at the national and international levels have brought not only fame and reputation to themselves, their parents, their families, the community, the school but also the province as well. These accomplishments could include the number of students who have received the title of 'National Good Student', the number of students who have won scholarships for overseas study in developed countries and have been accepted into national high-ranking higher education institutions, and have achieved many national or international awards. The principal stated that the accomplishments of individual students could often bring this school and province to the attention of the entire country. According to Thay Hoan, since the school was established, nearly all of the best and most qualified high school teachers from various schools around the province have been selected and appointed to teach at this school. For several years, those teachers have been recognized as experts in their subject knowledge and pedagogic teaching skills. They have a lot of experience and expertise in teaching good students for great achievements and awards. Several experienced teachers have retired in recent years, and PETS has chosen and recommended young university graduates with distinction qualifications for positions at the school. However, before accepting the PETS recommendation, this school has the right to interview and assess recommended graduates, and this is a priority for the school only. Those who have been recommended must take two examinations. The graduates' vocational main subjects are assessed in the first two-hour written examination. The second is about teaching methods. A school recruitment committee of three of the most qualified teachers will observe and evaluate each candidate's two lessons. The most qualified applicants are recruited as this famous high school's permanent staff.
Despite the fact that the teaching and learning quality level is better than in any other schools in this province, the teachers and principal continue to strive for more positive changes to meet the requirements of the current educational reforms initiated by MoET, the province authorities and the students' parents. "The team is dissatisfied with the current achievements," Thay Hoan said, "more TPD is required to enhance our knowledge and related skills on the basis of high-quality teacher input". Good students were both an advantage and a challenge at this school, as most teachers thought. Thus, they had to work harder to fulfill their students' educational expectations.
Both the principal and the teachers consider TPD as an essential aspect of the teaching career. This is "much more important for teachers in light of the latest general education reforms" (THI). Textbook replacement and teaching method reform are underway in Grade one of primary school, as well as grades six of secondary school and will be for Grade 10 of high school in the school year 2022-2023. Teachers must be well-prepared to deal with these changes in the near future: To master the new curriculum, textbooks, and teaching methods for improved teaching in the next few years, our teachers play an active part…New curriculum is associated with new teaching methods. Thus, in order to teach better, teachers must continually improve their professional capacities and update their new knowledge…If teachers are not provided with good professional development, they cannot keep up to date with the changes within the current reforms and therefore cannot satisfy requirements and fulfill their responsibilities. (THI) Overall, all the teachers provided similar explanations for the important roles of teacher professional development during this period. Almost all teachers seemed to agree that 'professional development is really significant for students' achievements in this school, particularly during the educational reform'. One female teacher with fifteen years' working experience made it clearer: As usual, we as teachers at this school also have to continually improve our knowledge in order to teach such intelligent and carefully selected students. During the educational reform, specific reforms of curriculum, textbooks, and teaching methods put higher requirements on teachers. Especially, new knowledge in textbooks and the new teaching method called the student-centered approach are really challenging us. We have to change, as a result, to improve our knowledge and skills. In order to do so, we should continue to have good professional development. Thus, professional development is most important to us as teachers. (Teacher questionnaire 1, hereafter referred to as T1)

Employment of strategies of teachers' professional development
It would appear that because of the realization of the significance of professional development for teachers' quality improvement, and ultimately for the achievements and improvements of students, especially during this period of reform, a variety of professional development strategies have been employed in this school. These strategies are divided into two groups: outside and inside the school. The outside activities involved degree upgrading, the PETS level good teacher competition, workshops or seminars organized by PETS or MoET. The school-based activities included peer observations, departments' weekly professional meetings, mentoring-coaching, and self-learning.
Degree upgrading: Degree upgrading was seen by the principal to be an effective strategy for young teachers. Young teachers often considered it a vital passport for their future in the teaching profession. However, in this school, there were older and more experienced teachers than young ones, because since the establishment of the school, experienced and good teachers, who were often middle-aged, were selected. In recent years, more young teachers have been employed to replace the retired ones. More and more teachers were asking the school management board to upgrade their degrees (to M.A or Ph.D. degrees). According to the above school report, around 58.6 percent of the teaching staff are Ph.D. or M.A holders of which 3 teachers have a PhD degree, and fifteen are doing the Ph.D. or M.A course. The principal noted that, "many teachers have wanted and registered to take M.A courses. I cannot allow many to take the course at the same time because there are not enough teachers to take their workload. Each year, from one to three are permitted to study. Older teachers are given priority" (THI). Hence, the need to do an M.A course among young teachers is high although the entrance examination is not easy. Every May or October, universities organize an examination for all registered teachers. High achievers are selected to be sent for Ph.D. or M.A courses. About 50 or 60 percent of all competitors are selected to take the course. The M.A course normally takes two years of full-time study, and the Ph.D. course takes four years in big cities, around 50-300 km from the city. Thay Hoan revealed that "I am happy that young teachers are eager to upgrade their certificates in spite of the difficulties they have to face during the course. Next year, three more teachers will be allowed to take the course".
He said that he had supported selected teachers in several ways. He assigned other teachers to teach their classes. Selected teachers also retained their salary during their time of study. They were also provided with money for travel and items purchasing. This support was recognized by all teachers in their questionnaires. One M.A holder wrote, "In general, the principal has created favorable conditions and encouraged me and other young teachers to do M.A courses. We highly appreciate this support" (T4). The school reports and documents indicated this support. For example, those teachers teaching studying teachers' classes were paid overtime wages.
'Good teacher' title competitions: Being a high school, teachers at this school can only register to be recognized as 'good teachers' at the provincial (PETS) level. After at least two years of having the school-level title of good teachers, these teachers are asked to register for higher titles. More specifically, a would-be 'good teacher' has to meet three criteria: passing the 'good teacher' competition every two years (both a written subject test and teaching lessons), having at least one good student at PETS level, and having a scientific study or experienced initiative recognized by PETS. According to Thay Hoan, more registered teachers in this school always attain the title because they have competitive advantages. First, the teachers' knowledge is generally much higher than other normal schools' because they are more carefully selected. Next, their students always get the PETS' student title because they are generally the most intelligent and capable students. Almost all the students of this school get the title, even 11 th grade students can get it or one student can get the title of two subjects (e.g., Maths and Chemistry), according to the school report. For example, in 2020, there were 950 students in total but there were 935 titles to be received. At present, according to Thay Hoan, 71 teachers are recognized as PETS level 'good teachers'. Despite these advantages, not all teachers are eager to register to compete for the title. According to many teachers, they were under pressure for the whole year if they competed for the title. One female teacher with 20 years of teaching experience wrote, "We have more advantages in the competition, really. But we are also stressed. We are teachers at this school; we have to try to get the highest scores and ranks to be respected" (T45).
Workshops and seminars: According to the principal, the school often created opportunities for its teachers to attend workshops or seminars or courses organized by MoET or PETS. Summer workshops were also obligatory for all teachers, as stipulated by MoET and PETS. During the school year, seminars or workshops were sometimes organized. When there was a notice of a conference, workshop, or course, the principal had a meeting with his deputy-principals to decide who, and how many, of the teachers would be assigned to go. After that, the department and deputy-principal arranged the time-table and asked other teachers to teach the selected teachers' classes. Teachers generally recognized their principal's support. One 43 year old female teacher wrote, "My principal tries to help us to attend workshops and seminars. We are also paid for travel and accommodation costs" (T80). One English teacher with five years of teaching experience noted, "When I participated in a three-week course in Ha Noi capital, my classes were taken by my colleagues. I could not do so without my principal's and colleagues' help and support" (T32). According to the principal, because the number of teachers attending those seminars or courses was restricted, those teachers were often required to share what they had learnt with other colleagues. Information sharing occurred in the weekly professional meetings. Even, "Those teachers can demonstrate by having real lessons for others to observe and reinforce the theory Teachers in the English group have done this well" (THI). We had opportunities to attend such information sharing weekly professional meetings at this school. In general, the principal author found teachers to be willing and active participants in such discussions.
Peer observations. Thay Hoan regarded peer observations as a strategy that was beneficial for his teachers' development. Each teacher was obliged to observe at least one lesson a week. In addition to individual observations, there were a number of special opportunities to have an observation. According to Thay Hoan, all of the teachers presented at least one lesson to be observed. Teachers generally realized the significant role of peer observation, and they all participated in this activity. Statements, such as "…[B]y observing other teachers' lessons, especially experienced ones, I can learn many things, teaching methodology in particular" (T8), "I can learn from my colleagues different ways of teaching a lesson that have helped me teach more interesting and better lessons" (T6), can be found in most teachers' questionnaires. Nevertheless, several teachers mentioned disadvantages. The first disadvantage for them was nervousness. This was especially evident among young teachers, when being observed. One female teacher with two years of teaching experience wrote:

…I often feel nervous when seeing many old and experienced teachers, especially the principal or deputyprincipals or PETS specialists, in the back seats looking at me. This is because I have never been confident in front of many people. Sometimes, my heart beats so violently that I forget some step in the lesson. It is not good when these lessons are observed and assessed for grades. I have tried some methods to decrease my nervousness. I don't look at my observers. I pretend to see them as other students in the class. And before starting teaching, I have a very deep breath to relax. I really feel much better. (T7)
The authors had a chance to observe two lessons at this school on the occasion of the birthday of the Communist Party. The authors also witnessed the teachers' nervousness and they spoke of this during the discussion after the observation. The second disadvantage was teachers' feelings during the discussion after peer observations. Most teachers mentioned their nervousness when receiving feedback from their colleagues. Thay Hoan understood this. According to him, teachers had an obligation to do peer observations. He tried to find ways to decrease their concerns. Teachers were made to be aware that this was one of their responsibilities and everyone did it in a supportive manner. "I told them that nobody is perfect. Peer observations make them grow and all feedback must be constructive, not seen as criticism… A friendly attitude towards one another is required in discussion after the observation…And these could help relieve pressure upon teachers" (THI). Four teachers noted their principal's encouragement and support in facilitating observations. Thay Hoan explained that observed lessons were graded and the teachers with the highest grades in each subject were invited to teach those lessons again for all other teachers in the departments to observe.
Weekly professional meetings. These were organized every Monday afternoon. According to Thay Hoan, teachers were divided into two major departments: Natural and Social sciences. The Natural Sciences department comprised the following subjects: Maths, Physics, Chemistry, Biology, Informatics, and English. The Social Sciences department included Literature, History, Geography, and Physical education. Sometimes, the departments were divided into groups of particular subject specialization areas such as Maths, or Physics. "Teachers can meet in their small groups to discuss their subject-related issues in detail" (THI). According to many teachers, in these meetings, they could exchange ideas such as 'a difficult Maths or Physics problem', 'good solutions for an international contest problem', 'an interesting poem, article, or novel". Thay Hoan stated that topic-based seminars were organized at the meetings. Teachers were expected to prepare ideas around the scheduled topics to discuss with their colleagues. In addition, the new information gathered from workshops or courses, organized by MoET, PETS, or projects, was shared with other teachers at these meetings. A teacher of English described this experience: Collaboration was reflected in this strategy and similar to what the TPD literature suggested (Fullan, 2011;Hallinger et al., 2021;Tran, Le, Nguyen et al., 2021).

I was assigned to attend a course on communication-based English language teaching method, organized by the
Our field notes document the above-mentioned issues. We witnessed what the teachers described above as well. However, not all professional meetings were as useful and interesting as the above story. Many teachers experienced topics that were boring and some teachers were not enthusiastic. One teacher wrote, "Some teachers did not prepare carefully for the topics for discussion. They appeared to come there just to be present. Some topics are difficult or unrealistic for teachers' everyday use. I think that this should be given more attention" (T5). Thay Hoan also expressed some concerns, "…Nevertheless, this [weekly professional meetings] is not as good as desired. Although this is beneficial and necessary for teachers when it operates effectively, we have not all done well. This is the strategy I am going to pay more attention to in the future" (THI). Our observation at such meetings also affirmed what the principal and the teachers mentioned.
Mentoring and coaching: This was effectively used in this school. Older and more experienced teachers were assigned by the principal to help and guide novice teachers mainly in professional knowledge, especially pedagogic teaching skills. Coaching was evident in other strategies such as peer observations and professional meetings. Teachers were "encouraged by the management board in general and the principal in particular to help one another develop and improve" (T6). Many teachers wrote that mutual understanding and personal relationships in this school also benefited their professional development in general and the coaching process in particular. Relationships between the principal and his colleagues, and between his teachers and teachers were built based on "mutual care and concern" (THI). These relationships were valued in Fullan's (2011) study. Thay Hoan further explained: As a tradition of Vietnamese people, we care for one another in professionalism and even in our social life.When a teacher is sick or having a difficult time such as accidents, parents' or children's or brothers' sickness or death, we often come to his/her house to offer condolence and an envelope of some cash as the school's encouragement. Depending on the level of events, we can have different kinds of support. For example, if the teacher's sadness is big, several teachers will visit his/her house more regularly to support and share with him/her losses and sadness. The motto 'tương trợ lẫn nhau' (mutual support' is strongly evident in this school. (THI) Self-learning: This was an important strategy for teachers at this school, as Thay Hoan revealed. According to Thay Hoan, this strategy definitely had a positive impact on teachers' knowledge. He indicated that "every year, all teachers have their own scientific studies or experienced initiatives that are highly appreciated by the school and PETS Science Councils" (THI). In addition, according to the principal, supervision and assessment of teachers' self-learning and scientific studies were carried out in several ways. First was through the teachers' 'accumulative professional knowledge notebook' and the study proposals. Second and more important, according to Thay Hoan, was through the results of those scientific studies and teachers' professional achievements (by their students' achievements). He elaborated: There are three alternative criteria for an assessment of students' achievements. First is the number of students passing the National upper-secondary graduation examination. The second indication is the number of students attaining an entry to university. The third is the number of good students at PETS, especially MoET level. These results or products can reflect a teacher's self-learning to a certain extent. (THI) In order to promote self-learning, Thay Hoan created conditions such as providing time and spiritual and material support. For example, teachers were encouraged to have their own shelf of books through the school providing a small amount of money for them to buy good books. Thus, many teachers were very interested in buying and reading good books. According to him, many teachers owned many valuable books that the school library had not purchased. Teachers indicated their passion for self-learning in their questionnaires. One female teacher with three years of teaching experience noted, Being a teacher of such gifted and clever students in this school has made me, as a young teacher, continually improve my knowledge, especially through self-learning. In order to teach a supplementary lesson for students † † , I have to read a variety of sources to find new knowledge for them. In general, besides their intellect, students of this school are hard-working and they have many books, even more than me. Thus, I have to read frequently. (T4) This was one of the reasons why the principal always encouraged his teachers to update their own shelves of books and materials. It appears that Thay Hoan used the instructional leadership style in this case. His teachers had autonomy in buying books and were empowered for self-learning. It was different from the leadership styles used by the principal to check teachers' notebooks in other cases.
According to the principal, all the above-mentioned strategies have operated relatively effectively. Peer observation was considered the most effective and useful for teachers' knowledge improvement both in terms of subject knowledge and teaching skills, Thay Hoan believed. The majority of the teachers, especially young ones, also valued this activity most. Our observation also highlighted this and noted that collaboration and mutual cooperation between teachers were evident in this activity.

The principal's support and role
Thay Hoan demonstrated a grasp of distinctions among teachers, including attitudes, academic levels, gender, and family circumstances. when answering the question, 'What are the differences between teachers in your opinion and how do your perspectives influence them?' "Apparently, those differences have different impacts on the process and effectiveness of teacher professional development. For example, one female teacher's needs, energy, and time for professional development are affected by her being busy with her family of small children or for parents' sickness" (THI). Many teachers remarked on empathy and understanding of their principal. A teacher of two years of teaching expressed,

I have been married and I am pregnant. So, I am often sick and tired. Thay [Hoan] seems to realize the situation. He does not ask me to be in charge of the Maths students' team, preparing for the next National Good Students' Competition. Instead, he asks another teacher to take the main responsibility. I am grateful for this and try my best to complete other tasks. (T7)
Thay Hoan demonstrated a spiritual intent and enthusiasm for providing TPD chances for his teachers to be improved, eventually benefiting his students, by employing a wide range of measures and creating conditions and mechanisms for promoting their TPD.
The principal has had a number of benefits and drawbacks when implementing the above-mentioned strategies. The most significant benefit, according to Thay Hoan, was that all teachers seemed to support the suggested solutions and tried their hardest to implement them. Second, the teachers' competence level was above average in comparison to those teachers in other high schools in the province, which was beneficial. This surely enhanced "facilitation of the professional development process" (THI).
In terms of challenges, Thay Hoan identified two major issues that have an impact on the level of professional development progress and effectiveness. The first was that teachers lacked a range of resources for their self-learning. The majority of the people did not know much English for their self-learning. As a result, "they [teachers] cannot access the plentiful sources from the Internet for their specialized subject knowledge improvement. They have missed an important tool to improve their knowledge" (THI). Teachers were well aware of this. "I know that if I had known English like Russian, I would have developed my knowledge even more" one teacher wrote, "Even though I am now in my senior years, learning English takes time and effort. This is for young teachers" (T89). According to Thay Hoan, a persistent issue was a shortage of funds. If he was provided more financial support, he could give more monetary rewards to his teachers and purchase more references and facilities for better teaching and learning. Money and other materialistic incentives were not mentioned in Western literature.
Thay Hoan explained that he had inspired his teachers by raising their awareness of the school's goals and educational reform, as well as their roles and status as educators. This demonstrated a shift in his thinking (Fullan, 2011). He also used a system of incentives, motivation, respect, and instructional supervision to inspire his teachers: I have to create a school environment in which there is trust, understanding, comfort, and motivation to appeal to teachers to enthusiastically and actively develop and devote their time to learning. Requirements and duties for teachers are clearly and openly articulated. I ensure them that I and other management board members are here to support them. (THI) The principal's sympathy and encouragement are known to most teachers. One teacher noted: Being a teacher at this school, we have many advantages and priorities. Our income is higher than our colleagues in other schools. Our prestige is higher… For example, every summer, the principal organizes a tour for all staff to visit different schools and places such as Da Lat, Ho Chi Minh city, and even Thai Land and China. The principal actively asks for sponsorship from businessmen, students' parents, the provincial authorities and so on. Thus, we are more aware of our responsibilities and duties to and for the school, and our students" (T79).
I reiterated the above statements to some degree and noted various items after spending nearly three months at the school and witnessing mutual communication and positive relationships between students, and between the principal and his teachers, as well as through questionnaires surveyed. The principal seemed to regard more senior and experienced teachers equally, and their relationships seemed to be similar. Young teachers seemed to pay respect to older teachers and more experienced teachers. I noticed a disparity in the principal's and older teachers' relationships. This distinction revealed the ways they spoke to each other in conversations at school.  (Field-notes, April 28, 2020) Nonetheless, the principal and teachers thought the relationships in this school were strong. Thay Hoan demonstrated the third aspect of Fullan's (2011) model, building relationships, by demonstrating a willingness to develop and maintain these relationships.
Finally, a number of strategies for teachers' professional development were used successfully to some degree, and there was some coherence between them. The principal was involved in and influenced the professional development of his teachers. One may argue that the principal's explained attitudes and behavior during that process demonstrated what other researchers noted.

Discussion
The principals and teachers in this high school were all aware of the value of TPD to the effectiveness of teaching staff. They all regarded teachers' professional development as very important for student learning and students' achievements. International research also related teachers' development and improvement to students' learning and achievement (Fullan, 2011;Hairon & Tan, 2017;Hallinger et al., 2021;Qian et al., 2017;Tran et al., 2018;Tran, Truong et al., 2021).
The principal realized that professional development for their teachers was an integral factor to the maintenance and promotion of quality teaching and learning. He mentioned a link between 'good teachers' and 'good students'. Thay Hoan, the principal, emphasized that relationship because his school is for such excellent students, and teachers have to train those students for many competitions in their specialized subjects. He considered TPD more important during the current educational reform. These seem to confirm what previous researchers found out (Ha & Murray, 2021;Hallinger et al., 2021;Sparks & Loucks-Horsley, 2018;Tran & Nguyen, 2019;Tran, Nguyen et al., 2020;Tran, Truong et al., 2020). They also argue that TPD is a primary vehicle in the effort to carry out the education reforms successfully.
Overall, almost all the teachers agreed with their principal. They were conscious of the important role of continual learning in their improvement and their job requirements. The teachers emphasized the need for professional development to allow them to meet the requirements and promote the success of educational reforms. Most teachers seemed to realize their strange position of being 'simultaneously both the subject and the agent of changes of the educational reforms' (Fullan, 2011;Qian et al., 2017;. The teachers all expressed a desire to learn the new teaching methods and understand the newly-printed textbooks. They generally wanted to master a learnercentered approach and apply it successfully to their classes to improve their students' capabilities.
Almost all teachers wrote of their own motivation before making a mention of requirements and request from MoET, Education Law, and their principal. Internal commitment was also highlighted in the international literature (Fullan, 2011;Sparks & Loucks-Horsley, 2018). It appears that the tradition of "tôn sư trọng đạo" (respecting teachers), the desire of preserving the honour of the teaching profession (Vasavakul, 2019), and maintaining and improving their own fame and prestige, have also motivated the teachers towards professional development. In general, the teachers regarded professional development as important for them because it could help them to develop tài in parallel with đức. An understanding of the causal relationship between 'good teachers' and 'good students' was evident in all the principals' interviews and almost all 82 teachers' questionnaires. These beliefs are consistent with Western literature Opfer & Pedder, 2011;Ping et al., 2018;Qian et al., 2017) and previous studies in the Vietnamese context (Hallinger et al., 2021;Tran et al., 2018;Tran, Nguyen et al., 2020;. This study identified and elaborated the range of seven activities employed in Ha Tinh Provincial High School for Gifted Students to promote its TPD. Both the scope and depth of learning activities seemed notable to the researchers. What also became apparent during the course of this research was the differentiated nature of these activities. More specifically, different activities appealed to these high school teachers based on their gender, age, experience, intellectual abilities and family situations. Hence, Thay Hoan was quite explicit about the wish to employ different strategies to motivate different teachers in their professional learning (Qian & Walker, 2013;Tonna & Shanks, 2017). The principal's role and support played a significant role in TPD.

Conclusion
In general, the principal and teachers recognized the importance of professional development for teachers in terms of improving teaching quality and increasing students' learning and achievement opportunities, especially at their high school for gifted students, it became more vital. Individual teacher experience, abilities, and dispositions are surely critical and can create positive impacts in individual classrooms (Ha & Murray, 2020Hallinger et al., 2021;Newmann et al., 2020;Tran et al., 2018;Tran, Le, Nguyen et al., 2021). Teachers' professional development was seen as critical by the principal and teachers in the school during the latest educational reform in Vietnam. In the principal's interview and almost all teacher questionnaires, there was a clear perception of the causal relationship between 'excellent teachers' and 'excellent students'. These viewpoints are supported by Western literature (Barth, 1990;Meissel et al., 2016;Ping et al., 2018;Prenger et al., 2017;Qian & Walker, 2013;Qian et al., 2017;Wang, 2016).
The principal's perspective on the significance of TPD on the school success and his students' progress and achievements have been transferred into real practice, including the policy enacted and a variety of strategies of TPD employed. This school employed seven strategies to develop teachers' professional expertise including three external ones and four school-based strategies. Generally speaking, those strategies were suitable for the teachers and they operated rather well. The teachers actively and enthusiastically participated in those strategies. Mentorship, open discussion sessions to deliberate on TPD issues, promoting collaboration by identifying and grouping teachers' interests, providing opportunities and monetary support for teachers to attend qualification-upgrading courses and workshops, and financial incentives and promotion as a motivational strategy were some of the solutions the deans supported TPD. It is noted that despite the hierarchical nature of leadership in Vietnam, these leadership practices have shown to be effective (He & Ho, 2017;Tran et al., 2018;Truong et al., 2016).
The principal's role and support were obviously significant in providing favorable conditions for TPD for the benefits of individual teachers, students' improvements, and the school's achievements. This result confirmed the previous research results (Hallinger et al., 2021;Lai et al., 2016;Tran et al., 2018;Tran, Nguyen et al., 2020;Tran, Truong et al., 2020).

Recommendations
There are a number of recommendations resulting from this study results. First, this study can bring about foundations in carrying out possible solutions to enhance the TPD quality at school and favorable conditions created by the principal for his teachers at a high school for gifted students in Ha Tinh province in particular and other high schools for gifted students in Vietnam in general. To improve gifted students' educational outcomes and achievements, the principal, his teachers and other educational authorities of Ha Tinh province should take measures to improve the quality of TPD. Furthermore, in the broader contexts of Vietnamese high schools for gifted students should be explored in future research. This research will provide important thoughts and implications for educational authorities as they explore methods to improve education quality in order to nurture talented students.

Limitations
There are some limitations of this study. To begin with, as with other qualitative case study studies, this was a single case study with a small and unique population of participants. The results of this study could not be directly applied to a wider group of people. Second, cultural and linguistic differences provided some obstacles in communicating the participants' and researchers' ideas. The original Vietnamese language meanings and contexts were more or less modified as a result of translation, although the semantics of the participants' original conceptions were remained faithful and consistent. There were no English equivalents for certain Vietnamese words or phrases in the English language. As Vasavakul (2019) suggests, when words are translated from one language and culture to another, it is difficult to tell if they have the same meaning for the people involved. Finally, while the case study method allowed the authors to conduct in-depth research on a single case, the conclusions may not be applicable for other Vietnamese high schools.