Although youth-led programs (YLP) have been successful in many areas of public health, youth leadership is rarely used in the prevention of peer aggression. A YLP to reduce bullying, sexual harassment, and dating aggression was compared experimentally with the board-mandated usual practice (UP). Four middle schools in an urban Canadian school division were randomly assigned to a YLP or to UP programs led by adults. Knowledge, attitudes, victimization, and emotional school adjustment (anxiety and school connectedness) were assessed in the fall (pretest) and spring (posttest) among 509 Grades 7 and 8 students (51.4% female, X(line over top) age = 12.37). Significant improvements were found in knowledge and attitudes in both programs. Students receiving the YLP showed significant reductions in anxiety and maintained their school connectedness (all ps < 0.05). The results suggest that youth-led prevention is an effective approach for tackling peer aggression in school settings.