Sexual violence among adolescents is a major concern. This study examined the impact of the peer-led "Sources of Strength" program on gender-based violence and attitudes dismissive of sexual harassment. Twenty high schools in Colorado were randomized to intervention (n = 11) or waitlist control (n = 9); students (N = 6461; 9th-11th graders) were surveyed four times (2017-2019); 45% Hispanic, 38% White, 2% Black, and 12% Multiracial; 78% straight/heterosexual, 11% bisexual, 4% questioning, 2% gay/lesbian, and 4% "other" sexuality. Baseline equivalence was not established for most outcomes. No direct intervention effects were found; however, self-reported exposure to "Sources" messaging was associated with small decreases in sexual harassment perpetration, forced sexual contact perpetration and victimization, and dismissiveness of sexual harassment in Year 2. Higher fidelity indicators were associated with exposure. School psychologists play key roles in identification and implementation of prevention programs, so suggestions to maximize implementation fidelity are provided.