This three-armed randomized controlled trial examined how moral disengagement and social norms account for change in bullying behavior and their potential as targets of anti-bullying components within separate interventions among 1200 French-speaking Belgian elementary students (48% boys, 9-12 year-olds, 57 classes, nine schools) during 2018-2019 (no ethnicity data available). Mediation analysis revealed that students' moral disengagement successfully decreased ([beta] = -0.46), which, in turn, reduced both bullying ([beta] = 0.33) and outsider behaviors ([beta] = 0.20), and increased defending ([beta] = -0.10). Intervening on social norms decreased bullying ([beta] = -0.18), but not through the perceived injunctive class norm as intended. Guidelines to open the "black box" of anti-bullying programs and determine the cost-effectiveness ratio of their components are provided.