The quality of sex education varies. In England from 2020, the government attempted to improve provision by making lessons a statutory requirement. We assessed implementation in 25 secondary schools in 2022–23, framed by May’s general theory of implementation. This identifies processes of sense-making, cognitive participation, collective action and reflexive monitoring, influenced by an intervention’s capability, stakeholder potential (individual and collective commitment), and institutional capacity (norms, and material and cognitive resources). Interview data from staff leading relationships and sex education (RSE) were coded thematically informed by May’s concepts. Those leading implementation ‘made sense’ of statutory guidance, finding it relevant and clear. ‘Cognitive participation’ among participants was strong, promoted by individual support for RSE but undermined by social norms prioritising academic attainment, limited skills among non-specialist teachers, and lack of ‘collective commitment’ among some staff and students. ‘Collective action’ varied across schools, influenced by availability of material resources and specialist staff. Schools undertook internal the ‘reflexive monitoring’ of provision, supported by school leaders’ awareness work would be assessed by government inspectors. On its own, statutory status is likely insufficient to achieve a step-change in RSE implementation. Other forms of support may be needed including training and offering support to more specialist teachers.