This longitudinal study examines the mediating mechanisms by which the KiVa antibullying program, based on the Participant Role approach, reduces bullying and victimization among elementary school students. Both student-level mechanisms leading to reduced perpetration of bullying and classroom-level mechanisms leading to reductions in bullying and victimization are considered. Analyses are based on a sample of 7,491 students (49.5% boys) nested within 421 classrooms within 77 schools. At the beginning of program implementation, the children were in Grades 4, 5, and 6 (mean age 11.3 years). Multilevel structural equation modeling was used to analyze whether changes in the hypothesized mediators accounted for later reductions in the outcomes. At the student level, antibullying attitudes and perceptions regarding peers' defending behaviors and teacher attitudes toward bullying mediated the effects of KiVa on self-reported bullying perpetration. The effects on peer-reported bullying were only mediated by antibullying attitudes. At the classroom level, the program effects on both self- and peer-reported bullying were mediated by students' collective perceptions of teacher attitudes toward bullying. Also, perceived reinforcing behaviors predicted bullying but did not emerge as a significant mediator. Finally, bullying mediated the effects of the classroom-level factors on victimization. These findings enhance knowledge of the psychosocial developmental processes contributing to bullying and victimization and shed light on the key mechanisms by which school bullying can successfully be counteracted.

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School bullying is increasingly viewed by researchers as a group phenomenon that extends beyond the perpetrator-victim dyad and is embedded in the wider social context. This paper reviews the literature on classroom and school factors contributing to bullying and victimization among children and adolescents. Considerable variability in the prevalence of these problems exists between classrooms and schools, which are highly relevant contexts for students' social development. Along with individual characteristics, both classroom-and school-related factors explain the bullying dynamic. The contexts may also exacerbate, or buffer against, the effects of individual-level risk for bullying involvement and the consequences of victimization. We discuss findings on the contributions of demographic and structural characteristics (e.g. grade level, classroom and school size), peer contextual factors (e.g. status hierarchy, group norms and bystander behaviours) and the role of teachers. Finally, implications for research and school-based antibullying programs are considered.

Over the last few decades, research on the role of classroom peer ecologies in student‐to‐student bullying has widened our understanding of this phenomenon. Bullying functions not only for individual perpetrators but also for the whole peer group by, for instance, providing a common goal and a semblance of cohesion for the group members. Bullying is more likely in classrooms characterized by poor climate, strong status hierarchy, and probullying norms. Bystanders’ responses contribute to the bullying dynamic by either rewarding or sanctioning the behavior of the perpetrators. Bystanders’ responses to bullying can be changed through school‐based programs, mediating the effects of the programs on bullying. Further efforts are needed to transform evidence into schools’ preventive practices encompassing the peer ecologies.

The aim of this study was to determine how trajectory clusters of social status (social preference and perceived popularity) and behavior (direct aggression and prosocial behavior) from age 9 to age 14 predicted adolescents' bullying participant roles at age 16 and 17 (n = 266). Clusters were identified with multivariate growth mixture modeling (GMM). The findings showed that participants' developmental trajectories of social status and social behavior across childhood and early adolescence predicted their bullying participant role involvement in adolescence. Practical implications and suggestions for further research are discussed.