Abstract Aulas en Paz (Classrooms in Peace) seeks to prevent aggression and promote peaceful relationships by means of a classroom curriculum for the development of socioemotional competencies; peer group sessions with two initially aggressive and four prosocial children; and workshops, home visits, and phone calls to parents. Preliminary evaluations in Colombian elementary schools have shown drastic reductions in aggression and substantial improvements in prosocial behavior, classroom climate, and friendship networks. The combination of universal components reaching all students and targeted components reaching more intensively those who need it the most appears to be responsible for such large effects.