This study aimed to investigate the efficacy of school-based prevention of suicidal behaviour programs. Participating schools were randomly assigned to one of three interventions or a control group. Interventions were (1) Question, Persuade, and Refer (QPR), a gatekeeper training module targeting teachers and other school personnel, (2) the Youth Aware of Mental Health Programme (YAM) targeting pupils, and (3) screening by professionals (ProfScreen) with referral of at-risk pupils. The primary outcome measure was the number of suicide attempt(s) made by 3 month and 12 month follow-up. No significant differences between intervention groups and the control group were found at the 3 month follow-up. At the 12 month follow-up, YAM was associated with a significant reduction in suicide attempts and severe suicidal ideation. These findings underline the benefit of this universal suicide preventive intervention in schools.