This study aims at investigating forms of aggressive behaviors occurred at SMK Muhammadiyah Mungkid Magelang, the role of peers in the occurrence of aggressive behaviors, and the factors causing aggressive behaviors. This study implemented qualitative approach. The subjects of the research were Islamic Studies teacher, School Counsellor, and the students involved in aggressive behavior. The research data were obtained from interview, observation, and document. The interview was conducted using snowball sampling method. The data were analyzed using the data analysis theory proposed by Mileas and Huberman which included data reduction, data display, and conclusion drawing. The results of the study were: (1) Aggressiveness happened at SMK Muhammadiyah Mungkid were in the form of brawl identified since 2012. The brawl involved the students with some students of other school. One thing which significantly supported and influenced the behavior was students’ peers. 2) The factors causing aggressive behaviors were: a) recruitment of brawl group members at SMK Muhammadiyah Mungkid Magelang, b) invitation from students of other school, and c) peers alliance. 3) The forms of aggressive behaviors were: a) destruction of facilities, b) usage of weapon and sharp objects, and c) harming others. 4) The behavior resulted in the people’s low of trust in SMK Muhammadiyah Magelang. In addition, a number of tenth grade students are recruited to join the brawl group each year.