In the last two years, a growing number of studies have focused on the promotion of students' mental health to address the negative effects of the COVID-19 pandemic. However, less studies have been conducted on sustaining teachers' mental health which has been affected by the sudden changes in online teaching and the difficulties in keeping and building relationship with students. Even before the pandemic, teaching has long been recognised as one of the most challenging occupations characterized by high levels of stress. Although the research highlighted the key role of mental health promotion among teachers, there is still a lack of programs enhancing teachers' wellbeing. This study examined the impact of the PROMEHS program, a school-based curriculum, on teachers' mental health. A total of 687 teachers participated in the study. Applying a pre- and post-training study design with experimental and waiting list groups, teachers were evaluated in social and emotional learning, resilience, and self-efficacy. The results showed that there was a significant improvement in all competences of the teachers in the experimental group compared to those in the waiting list group. The paper discusses the implications of the findings with recommendations for further studies in the area.