In recent years, increase in adolescent crime in the Republic of Korea has put adolescent aggression in the spotlight. This study examines whether the quality of attachment to parents and peers influences aggressive behaviors and whether social emotional competencies serve as significant mediators for middle school students. These relationships were investigated between gender groups. A full mediational model better explained the male sample while a partial mediational model better explained the female sample. Specifically, parent and peer attachment was found to have indirect effects on aggression via behavioral self-control among boys, whereas for girls, parent attachment was found to have both direct and indirect effects on aggression via empathy and behavioral self-control. The findings of the study underscore the importance of parent and peer attachment during adolescent years. Significant implications for teachers and school psychologists as well as aggression-prevention program developers are discussed.