BACKGROUND: Systematic reviews suggest that interventions that address school organisation are effective in reducing victimisation and bullying. We successfully piloted a school environment intervention modified from international studies to incorporate 'restorative justice' approaches. This trial aims to establish the effectiveness and cost-effectiveness of the INCLUSIVE intervention in reducing aggression and bullying in English secondary schools. DESIGN: cluster randomised trial. PARTICIPANTS: 40 state-supported secondary schools. OUTCOMES assessed among the cohort of students in year 8 (n = approximately 6,000) in intervention year 1. INTERVENTION: INCLUSIVE is a school-led intervention which combines changes to the school environment with the promotion of social and emotional skills and restorative practices through: the formation of a school action group involving students and staff supported by an external facilitator to review local data on needs, determine priorities, and develop and implement an action plan for revising relevant school policies/rules and other actions to improve relationships at school and reduce aggression; staff training in restorative practices; and a new social and emotional skills curriculum. The intervention will be delivered by schools supported in the first two years by educational facilitators independent of the research team, with a third locally facilitated intervention year.Comparator: normal practice. OUTCOMES: primary: 2 primary outcomes at student level assessed at baseline and at 36 months:1. Aggressive behaviours in school: Edinburgh Study of Youth Transitions and Crime school misbehaviour subscale (ESYTC)2. Bullying and victimisation: Gatehouse Bullying Scale (GBS)Secondary outcomes assessed at baseline, 24 and 36 months will include measures relating to the economic evaluation, psychosocial outcomes in students and staff and school-level truancy and exclusion rates. SAMPLE SIZE: 20 schools per arm will provide 90% power to identify an effect size of 0.25 SD with a 5% significance level.Randomisation: eligible consenting schools will be randomised stratified for single sex versus mixed sex schools, school-level deprivation and measures of school attainment. DISCUSSION: The trial will be run by independent research and intervention teams and supervised by a Trial Steering Committee and a Data Monitoring Committee (DMC). TRIAL REGISTRATION: Current Controlled Trials ISRCTN10751359 (Registered 11 March 2014).