Evaluation of a partially contextualized bullying prevention program for first-year upper-secondary students: study protocol for a quasi-experimental controlled trial

Bullying in adolescence increasingly takes more indirect forms, making it harder to detect and address. Yet most prevention programs remain tailored to younger students, thereby overlooking the developmental and social complexities of mid-adolescence. To be effective, interventions need to align with age-specific peer dynamics and target key mechanisms such as classroom norms, bystander roles, and moral disengagement. Preventive efforts should be implemented early during the transition to a new educational level, when students enter newly formed classes and classroom norms and social hierarchies are still emerging. This study protocol introduces a quasi-experimental study aimed at investigating a developmentally grounded, partially contextualized program for first-year upper-secondary students, targeting both traditional and cyberbullying within newly formed classroom groups. The intervention includes twelve sessions, comprising six standardized core sessions, two class-specific tailored sessions, and four sessions dedicated to assessment procedures. A total of 180 students from six classes (three intervention and three control) participate in the study. Data are collected at baseline, post-intervention, 4-month follow-up, and one-year follow-up using validated measures of bullying, classroom climate, bystander behavior, moral disengagement, and peer norms. After each session, students complete a brief evaluation questionnaire. Teachers, school staff, and parents receive preparatory training. Planned analyses include mixed-design analyses of variance (ANOVAs), with effect sizes reported as partial eta squared ( η 2 p), as well as thematic analysis of student feedback. The intervention is expected to yield both immediate and sustained reductions in bullying and victimization among first-year upper-secondary students. Improvements are also anticipated in classroom climate, including greater peer support, reduced deviant behavior, and stronger prosocial norms, alongside increased defending behavior, lower moral disengagement, and a shift away from aggressive popularity norms within the peer group.