OBJECTIVES: Bullying and aggression among children and young people are key public mental health priorities. In this study, we evaluated the cost-effectiveness of a complex school-based intervention to address these outcomes within a large-cluster randomized trial (Inclusive). METHODS: Forty state secondary schools were randomly allocated (1:1) to receive the intervention or continue with current practice as controls. Data were collected using paper questionnaires completed in classrooms including measures of their health-related quality of life using the Childhood Utility Index and police and National Health Service resource use. Further detailed data were collected on the cost of delivering the intervention. We calculated incremental cost-effectiveness ratios following the intention-to-treat principle using multilevel linear regression models that allowed for clustering of pupils at the school level. RESULTS: Overall, we found that the intervention was highly cost-effective, with cost-per quality-adjusted life year thresholds of £13 284 and £1875 at 2 years and 3 years, respectively. Analysis of uncertainty in the result at 2 years revealed a 65% chance of being cost-effective, but after 3 years there was a 90% chance that it was cost-effective. CONCLUSION: This study provides strong evidence collected prospectively from a randomized study that this school-based intervention is highly cost-effective. Education- and health-sector policy makers should consider investment in scaling up this intervention.