Teachers play a fundamental role in the promotion and successful implementation of school health services. This study aimed to assess the level of involvement in the Health Promoting School program and its associated factors and to explore the benefits and barriers to involvement among high school teachers in Myanmar. A mixed methods explanatory equential study was conducted among 194 high school teachers in Thanlyin Township, Yangon Region, Myanmar, from June to August 2020. Quantitative data were collected with the pretested structural questionnaire and analyzed by Chi-square tests and Fisher’s exact tests. A qualitative strand was added by conducting in-depth interviews (n = 15, five teachers from each level of involvement: poor, medium, and good), analyzed by thematic content analysis. Of the 194 teachers, 23.7% had a good level of involvement in the Health Promoting School program. The factor associated with involvement in Health Promoting School program were age (p value < 0.001), duration of services (p value = 0.001), and a number of accomplished training-related school health (p value = 0.008). Qualitative data revealed that improvement of the health knowledge and awareness on health problems, the progress of healthy behaviors, development of physical and mental health, prevention of the disease spread, achievement of healthy and productive learning environment, and development of academic achievement were major benefits of teachers’ involvement. Moreover, the main barriers to involvement were insufficient materials and human resources, time constraints, incompetence of the teachers, poor cooperation of school health partnerships, and insufficient
awareness of parents. The proportion of good involvement in the Health Promoting School program among high school teachers was low in this study area. Providing sufficient human resources and material, conducting the on-the-job and refresher training, enhancing parent-teacher cooperation, and strengthening the community partnerships were crucial to improve the level of involvement and reduced the barriers for the achievement of the Health Promoting School program.