The novel approach to studying peer victimization is to focus on the harm resulting from aggression rather than a power imbalance between victim and bully, especially since various forms of aggression, which do not meet bullying criteria, can be equally harmful. The main aim of the study is to identify the most harmful act of aggression that is experienced by Polish adolescents. Secondary goals are to check how the various forms of victimization are connected and to verify whether experiencing harm is related to adolescents’ well-being. The group of 551 Polish adolescents participated in the study which was conducted in four public schools. The study was designed as correlational research and the main variables – harmful aggressive behaviors and well-being – were measured using the Student Aggression and Victimization Questionnaire and Mental Health Continuum Scale – Short Form, respectively. From 10 presented aggressive behaviors the most harmful is being left out. The harm resulting from rejection is negatively correlated with adolescents’ well-being. Additionally, pupils who experience being left out are not only victims but also perpetrators of other acts of violence. The sociopolitical background and applications of our findings are discussed.