This research involved a three-year cluster randomised control trial to develop, implement and evaluate the Friendly Schools intervention which aimed to reduce bullying and aggression among more than 3,000 students who had recently transitioned to secondary school. Intervention schools were provided with individualised training and resources to support students’ transition and reduce bullying using a multi‐level comprehensive intervention addressing classroom curriculum, school policies and procedures, the social and physical environment, pastoral care approaches and school‐home‐community links. Although the observed effect sizes were small, the intervention had a consistently significant positive effect across a range of outcomes, including bullying perpetration, victimisation, depression, anxiety, stress, feelings of loneliness and perceptions of school safety at the end of the students’ first year in secondary school. However, none of these differences were sustained into the students’ second year of secondary school. It is clear that strategies to reduce bullying need to continue to support students as throughout their schooling to sustain benefits.