Adolescence is a key time to prevent or reduce poor mental health outcomes. Supportive school environments play an important role in this, and the concept of health-promoting schools have been supported globally. Participatory action research (PAR) combines theory, practice, action, and reflection by developing practical solutions to address concerns and issues within communities. Running four PAR groups across three secondary schools, we explored the feasibility of using the approach as a mechanism for bringing about culture change and improving mental health. We undertook interviews and focus groups with students (n = 24), school staff (n = 11), facilitators (n = 3), and parents/carers (n = 2). Findings are organised under five key headings: 1) Establishing PAR groups, and the PAR cycle; 2) PAR group impact; 3) Facilitators of PAR success; 4) Barriers to PAR success; 5) Future recommendations. This study demonstrated the feasibility of PAR as a tool to improve school culture. Students participating in PAR were engaged, passionate, and motivated to influence and transform school culture to improve mental health. Future research should aim to trial the PAR approach on a larger scale, to determine whether the barriers and facilitators of PAR success identified here are relevant and transferable to schools in other contexts, and to measure the impact of such initiatives on mental health outcomes.