Defending in bullying is a complex, yet important behavior that is likely associated with individual characteristics and group factors that operate simultaneously in the classroom microsystem. However, little research has longitudinally analyzed the role of multiple promoting factors at both the individual and classroom level. Drawing on the social-ecological theory and social-cognitive theory, the present study examined the prospective associations between Fall defending self-efficacy, moral disengagement, moral identity, and moral distress and Spring defending behavior. Participants were 1163 adolescents (48.7% females; M = 13.6, SD = 1.1) attending 67 classrooms in Italian public schools. Defending showed moderate stability over one school year. At the individual level, multilevel analyses showed that T1 self-efficacy for all students, and moral distress for male students, positively predicted T2 defending. Moreover, high moral disengagement negatively predicted T2 defending only when students also reported high levels of moral identity. At the class-level, T1 class defending and class moral identity explained between-class variability in T2 defending. The findings have multiple implications for interventions that aim to increase defending behavior.