This one-year, three-wave longitudinal study adopted a positive prevention approach to test whether a school-based mental health intervention helped avert declines in student and teacher wellbeing at a P-12, co-educational school in Hong Kong, Special Administrative Region of the People's Republic of China (Hong Kong SAR, China). Teachers (n = 181) and students (n = 782) rated their individual levels of wellbeing at three time points (May 2021, September 2021, May 2022) and were also asked rate the levels of collective wellbeing they saw in others at the start and end of the intervention (May 2021 -May 2022). A comparison of the degree to which students felt their teachers were implementing wellbeing strategies in the classroom before and after the intervention was also made. Self-ratings of wellbeing at three time points remained stable for teacher and students, suggesting that the intervention played a protective role in wellbeing by preventing a decline. Additionally, the experience of high use of the wellbeing intervention strategies in class was associated with an increase in students' wellbeing across time during the pandemic. Students saw the collective wellbeing of their peers grow significantly from the start to the end of the intervention but reported no change in the level of collective wellbeing they saw in their teachers. According to the teachers, the collective wellbeing of their students and colleagues significantly increased over time. Levels of collective wellbeing were significantly correlated with individual wellbeing suggesting the whole-school approach to the intervention was valuable. Implications point to the importance of designing whole-school, context-based interventions that include all stakeholders and, thus, allows both individual-level and collective-level wellbeing to improve.