Peer bullying in schools is a significant public health problem that contributes to poor health and wellbeing outcomes for those who bully or are bullied. Meta-analyses of the efficacy of secondary school bullying prevention interventions have typically found no effects or an increase in student bullying. Consequently, few secondary school studies have examined the “real-world” effectiveness of these interventions. This age-cohort study design evaluated the effectiveness of the Friendly Schools (FS) secondary school intervention, previously found to be efficacious. FS was implemented in schools under real-world conditions by an education publisher. Student survey data were collected in 12 schools. The primary outcomes were bullying victimisation and perpetration. Results showed a significant decrease in reported bullying perpetration in subsequent cohorts of both grade 8 and 9 students, and a significant reduction in bullying victimisation and cybervictimisation for grade 8 students, when the FS student curriculum was taught compared to the usual curriculum. This study demonstrates the importance of considering the effectiveness of secondary school bullying prevention interventions and real-world implementation supports for schools.