Esther Mertens

| 31 Study Protocol and teachers. The interim measurements (during R&W) are shortened questionnaires and are completed by students after a series of three R&W lessons. Deviant and prosocial communication is assessed in the first year prior and after R&Wusing video-observations in a subsample. Moderators (i.e., student, teacher, and parent characteristics) are assessed at one time point. The study is registered with the Dutch Trial Register (6554) and has been approved by the Ethical Committee of the Faculty of Social and Behavioral Sciences of Utrecht University (FETC17-015). Study sample In total, we aim to include 720 students in this study across all waves, 180 per condition. Participants are students in the 7th Grade, in the second year of the intervention in the 8th Grade, of preparatory vocational education level. Schools are excluded if they currently implement R&W in the whole school or have implemented the intervention in the past two years in the whole school. Additionally, schools for students with special needs are excluded from the study. Recruitment Schools were recruited through the network of the developers of R&W, the Gadaku Institute. An e-mail to certified R&W trainers was sent with information about this study and a message on the private R&W online forum was posted. Trainers asked within their network whether schools that did not implement R&Wwere interested in participating in this study. When schools were interested they contacted the Gadaku Institute which referred the school to the researchers. The researchers established whether the school was eligible for participation. If the school was eligible, the researcher provided additional information to the schools based on which the school made a decision concerning participation. Schools were randomly assigned (1:1:1:1 ratio) by stratified block randomization, with blocks of four (i.e., the number of conditions in this study). Schools were stratified by school size (small to moderate sized schools: < 100 students in 7th Grade, large sized schools: > 100 students in 7th Grade) to establish a more equal distribution of students over the four conditions. The randomization numbers were generated by a random number generating computer program. Students, parents, teachers, and R&W trainers received an information letter to inform themabout R&Wand the study. Parents also attended a parent-teacher evening where they received additional information about R&W. Students gave active informed consent and parents passive informed consent for participation of the student. Parents gave active informed consent for their own participation in the study. Active informed consent was also acquired for participation of teachers and R&W trainers. 2