Purpose: This three-year group randomized controlled trial assessed whether a multi-age, multi-level bullying prevention and intervention with staff capacity building, can reduce bullying among primary school children. Methods: This study comprised two intervention and one comparison conditions. Student self-report data were collected from 2552 Grades 4 and 6 students at baseline and three posttests. Results: The high intervention was more effective than the moderate and low interventions, with significant positive effects for "being bullied" in the Grades 4 and 6 cohorts, for "bullying others" in the Grade 4 cohort and "telling if bullied" in the Grade 6 cohort. Conclusions: Comprehensive whole-school programmes that include capacity building and parental involvement appear to reduce bullying behaviour more than programmes without these components. (Contains 1 figure and 8 tables.)