The aim of the present study was to examine whether class-level characteristics, such as class size and gender ratio, could contribute to explaining different levels of bullying, victimization, and perception of defending between classes. Seven-hundred-and-sixty-three students (M[subscript age] = 12.96, SD = 1.22; 54.4% boys) from 42 Portuguese middle school classes participated in the current study. Bullying, victimization, and perception of defending were assessed through a self-report questionnaire. In order to account for the clustered nature of the data with students nested within classes, multilevel models were used. After controlling for age and gender at the individual level, gender ratio contributed to explaining between-class variance in bullying and victimization, while class size was a significant predictor of the perception of defending. Thus, the results suggested that both bullying and victimization are more frequent in classes with a high concentration of boys, while in larger classes there is a higher perception of defending. The results of the present study support the importance of taking into account class-level characteristics when developing anti-bullying interventions, and of establishing gender balanced classes in middle school. The results also highlight the relevance of using multilevel modelling when conducting research on bullying and victimization in schools.