Background ‘Dating and relationship violence’ is intimate partner violence during adolescence. Among dating adolescents in England, 66–75% of girls and 32–50% of boys report victimisation. Multicomponent school-based interventions might reduce dating and relationship violence. We optimised and piloted Project Respect, a new intervention in secondary schools in England, and study methods, to assess the value of a Phase III randomised controlled trial. Objectives To optimise Project Respect and to then conduct a pilot randomised controlled trial in southern England, addressing whether or not progression to a Phase III trial is justified in terms of prespecified criteria. To assess which of two dating and relationship violence scales is optimal, to assess response rates and to consider any necessary refinements. Design Optimisation activities aimed at intervention development and a pilot randomised controlled trial. Setting Optimisation in four secondary schools across southern England, varying by region and local deprivation. A pilot cluster randomised controlled trial in six other such schools (four intervention schools and two control schools), varying by region, attainment and local deprivation. Participants School students in years 8–10 at baseline and staff. Interventions Schools were randomised to the intervention or control arm in a 2 : 1 ratio; intervention comprised staff training, mapping ‘hotspots’ in school for dating and relationship violence, modifying staff patrols, school policy review, informing parents and carers, an application supporting student help-seeking, and a classroom curriculum for students in years 9 and 10 (including student-led campaigns). Main outcome measures Prespecified criteria for progression to Phase III of the trial, concerning acceptability, feasibility, fidelity and response rates. Primary health outcomes were assessed using the Safe Dates and short Conflicts in Adolescent Dating Relationships Inventory measures collected and analysed by individuals who were masked to allocation. Feasibility of economic analysis was assessed. Data sources Baseline and follow-up student and staff surveys, interviews, observations and logbooks. Results The intervention was optimised and approved by the Study Steering Committee. The student response rates in intervention and control groups were 1057 (84.8%) and 369 (76.6%) at baseline, and 1177 (76.8%) and 352 (83.4%) at follow-up, respectively. Safe Dates and the short Conflicts in Adolescent Dating Relationships Inventory had high levels of completion and reliability. At follow-up, prevalence of past-year dating and relationship violence victimisation was around 35% (Safe Dates scale and short Conflicts in Adolescent Dating Relationships Inventory). Staff response rates were very low. Training occurred in all four schools, with suboptimal fidelity. The curriculum was delivered with optimal fidelity in three schools. Other components were delivered inconsistently. Dating and relationship violence was addressed in control schools via violence prevention and responses, but not systematically. Intervention acceptability among students and staff was mixed. An economic evaluation would be feasible. Limitations One school did not undertake baseline surveys. Staff survey response rates were low and completion of the logbook was patchy. Conclusions Our findings suggest that progression to a Phase III trial of this intervention is not indicated because of limited fidelity and acceptability. Future work High prevalence of dating and relationship violence highlights the ongoing need for effective intervention. Potential intervention refinements would include more external support for schools and enhanced curriculum materials. Any future randomised controlled trials could consider having a longer lead-in from randomisation to intervention commencement, using the short Conflicts in Adolescent Dating Relationships Inventory as the primary outcome and not relying on staff surveys. Trial registration Current Controlled Trials ISRCTN65324176. Funding This project was funded by the National Institute for Health Research (NIHR) Public Health Research programme and will be published in full in Public Health Research; Vol. 8, No. 5. See the NIHR Journals Library website for further project information.