Background: The sustainment of school-based interventions to improve students’ behaviour, health and wellbeing has been relatively unexplored compared to healthcare interventions. Discontinuing effective interventions prevents new practices from reaching new student cohorts and wastes implementation investment. This study examines the sustainment and adaptation of a school intervention to improve risk behaviours that aimed to enhance sustainability by encouraging school commitment, ownership and local adaptation.Method: A case study was conducted of the sustainment of ‘Learning Together’, an intervention to reduce multiple risk behaviours initiated in English secondary schools through an effectiveness trial. The intervention entailed restorative practice (RP), a staff-student action-group and a curriculum. Qualitative, longitudinal data were collected from five schools: interviews with multiple staff per school, and with students and external facilitators the first-year post-trial; interviews with one staff member per school two years post-trial; and descriptive data from the trial’s original process evaluation.Results: Learning Together, as a whole intervention, was not sustained two years post-trial. RP was the most successfully sustained component; all staff interviewed continued to use RP in some form in their individual practice in years 4 and 5 and was sustained at school-level in one school. The curriculum and action-groups were discontinued in all schools, though actions initiated by the groups were sustained in two schools. Staff were motivated to sustain an approach at an individual-level if they perceived it as more effective than existing practices at improving students’ wellbeing or behaviour and it was practically achievable. Elements designed to build commitment, ownership and local adaptation were largely ineffective at sustaining components, as sustainment at school-level required ongoing attention to changes to organisational practices, policies and systems, which was beyond the remit of individual staff. Conclusion: Schools need greater support from intervention developers to sustain interventions at school-level. Adaptation could help or hinder sustainment; schools need support to adapt components according to their theoretical rationale and develop different implementation options. Further methodologically strong primary research on sustainment and sustainability strategies is needed.