School-based interventions for preventing dating and relationship violence (DRV) and gender-based violence (GBV) are an important way of attempting to prevent and reduce the significant amount of DRV and GBV that occurs in schools. A theoretical understanding of how these interventions are likely to cause change is essential for developing and evaluating effectiveness, so developing an overarching theory of change for school-based interventions to prevent DRV and GBV was the first step in our systematic review. Theoretical data were synthesised from 68 outcome evaluations using methods common to qualitative synthesis. Specifically, we used a meta-ethnographic approach to develop a line-of-argument for an overarching theory of change and Markham and Aveyard's (2003, Social Science & Medicine, 56, 1209) theory of human functioning and school organisation as a framework for structuring the concepts. The overall theory of change generated was that by strengthening relationships between and among staff and students, between the classroom and the wider school, and between schools and communities, and by increasing students' sense of belonging with student-centred learning opportunities, schools would encourage student commitment to the school and its values, prosocial behaviour and avoidance of violence and aggression. The theory of human functioning informed our understanding of the mechanisms of action but from our analysis we found that it required refinement to address the importance of context and student agency.