Education policy increasingly promotes action groups as a key strategy for student and/or staff participation in school improvement and whole‐school health promotion. Such groups can coordinate multi‐component interventions, increase participation and engagement, and enable local adaptations, but few process evaluations have assessed this. We evaluated fidelity, feasibility and acceptability of action groups as part of a trial of a whole‐school intervention to reduce bullying and aggression and promote health in English secondary schools, which reported multiple health and educational impacts. Action groups involved students and staff, supported by external facilitators, and drew on data on student needs. They aimed to: coordinate implementation of restorative practices and a social and emotional competencies curriculum; review policies and rules; and enact local decisions to modify school environments. Our process evaluation used interviews, focus groups, observations and questionnaires to assess action groups’ fidelity, role in coordination, role in local adaptation, support from external facilitators and data on student needs, and acceptability in engaging members. Fidelity was high in the first two years but lower in the third year when external facilitators withdrew. Student needs data were perceived as useful, but views on external facilitators were mixed. Groups successfully reviewed policies and rules, planned activities and coordinated restorative practices, but were less successful in implementing the curriculum. Success was facilitated by the involvement of school leaders. Members reported high satisfaction and empowerment. Action groups are a promising strategy for leading whole‐school health promotion. Implementation is supported by external facilitation, local data and involvement of senior managers.