INTRODUCTION: Youth suicide rates have risen steadily in recent decades, with suicide attempts representing a significant risk factor for future mortality. Sexual violence exposure further elevates this risk. This study investigated the efficacy of Sources of Strength, a school-based peer-led program, for reducing suicide attempts school wide as well as specifically among youth with recent sexual violence exposure. STUDY DESIGN: Cluster RCT was conducted in 2017-2019, contrasting 22 schools paired and then randomized to either Sources of Strength intervention (n=11) for 2 school years or wait-listed condition (n=11). Two control schools dropped out prior to completing baseline assessments. SETTING/PARTICIPANTS: Participants were 6,539 students in 20 high schools across 4 assessment waves: baseline prior to random assignment and 6-, 12-, and 18-month follow-ups. INTERVENTION: In schools assigned to immediate Sources of Strength implementation, student peer leaders (n=348) received training in the program's multidimensional healthy coping framework in fall of both school years, and peer leaders conducted school-wide campaigns with ongoing mentoring from staff adult advisors (n=122). MAIN OUTCOME MEASURES: The primary outcome was any new suicide attempt reported at Waves 3 and 4. Recent sexual violence history was also assessed at baseline. Analyses were conducted in 2023. RESULTS: Sources of Strength significantly reduced new suicide attempts by 29%, even after accounting for student characteristics and recent sexual violence history. Moderation analyses showed that this overall intervention effect may not have extended to students with more severe sexual violence victim histories. CONCLUSIONS: Sources of Strength effectively reduced suicide attempts in high-school students overall, suggesting its potential for population-level impact. This is especially important in the face of steadily rising suicide attempt and fatality rates among adolescents and young adults. However, a cautious interpretation of the evidence suggests that Sources of Strength may not prevent attempts among students with recent sexual violence contact victimization, highlighting the need for additional intervention work for this high-risk group. TRIAL REGISTRATION: This trial is registered at Clinical Trials.gov NCT03014271.