AbstractTeaching staff report poorer mental health and wellbeing than the general working population. Intervention to address this issue is imperative, as poor wellbeing is associated with burnout, presenteeism, and adverse student mental health outcomes. The Wellbeing in Secondary Education (WISE) intervention is a secondary school-based programme aimed at improving the mental health and wellbeing of teachers and students. There are three components: awareness-raising for staff; a peer support service delivered by staff trained in Mental Health First Aid (MHFA); and Schools and Colleges Mental Health First Aid (MHFA) training for teachers. A cluster randomised controlled trial with integrated process and economic evaluation was conducted with 25 secondary schools in the UK (2016–2018). The intervention was largely ineffective in improving teacher mental health and wellbeing. This paper reports process evaluation data on acceptability to help understand this outcome. It adopts a complex systems perspective, exploring how acceptability is a dynamic and contextually contingent concept. Data sources were as follows: interviews with funders (n = 3); interviews with MHFA trainers (n = 6); focus groups with peer supporters (n = 8); interviews with headteachers (n = 12); and focus groups with teachers trained in Schools and Colleges MHFA (n = 7). Results indicated that WISE intervention components were largely acceptable. Initially, the school system was responsive, as it had reached a ‘tipping point’ and was prepared to address teacher mental health. However, as the intervention interacted with the complexities of the school context, acceptability became more ambiguous. The intervention was seen to be largely inadequate in addressing the structural determinants of teacher mental health and wellbeing (e.g. complex student and staff needs, workload, and system culture). Future teacher mental health interventions need to focus on coupling skills training and support with whole school elements that tackle the systemic drivers of the problem.