Esther Mertens

| 35 Study Protocol Instruments An overview of the concepts, instruments, measurement points and informants is presented in Additional file 1, Table 2. Socio-emotional adjustment To assess psychosocial wellbeing students, parents and (non-trainer) teachers complete the short version (12 items; e.g., “I am stubborn.”) of respectively the Youth Self Report (YSR; Achenbach, 1991b; Verhulst, Van der Ende, & Koot, 1997b), the Child Behavioral Checklist (CBCL; Achenbach, 1991a, Verhulst, Van der Ende, & Koot, 1996) and the Teacher’s Report Form (TRF; Achenbach, 1991c; Verhulst, Van de rEnde, & Koot, 1997a) based on the study of Chorpita and colleagues (2010). Additionally, students and parents fill in the subscale Psychological wellbeing (7 items; e.g., “Did you had fun?”) of the KIDSCREEN-27 (Ravens-Sieberer & The European KIDSCREEN Group, 2006). Sexual autonomy is reported by the students. For this, items from a national study in The Netherlands concerning sexual health (De Graaf, Meijer, Poelman, & Vanwesenbeeck, 2005) are used. These 5 items represent interaction competence concerning control, assertiveness and self-esteem (e.g., “I have little influence on what happens.”). To measure resilience, students, parents, and (non-trainer) teachers complete the Connor-Davidson Resilience Scale short version (CD-RISC 10; Campbell-Sills & Stein, 2007). The 10 items reflect the self-beliefs to cope with difficulties in life (e.g., “Able to adapt to change.”). Social safety Perceived social security in the classroom is measured using the subscales Comfort (4 items; e.g., “In this class, I can be myself.”), Conflict (4 items; e.g., “In this class, children argue with each other.”), and Cohesion (4 items; e.g., “In this class, everyone likes each other.”) of the Classroom Peer Context Questionnaire (Boor-Klip, Segers, Hendrickx, & Cillessen, 2016) completed by students and (non-trainer) teachers. It assesses the perception of school culture, for instance how positive, respectful, friendly and helpful students are towards each other and sense of belonging. Aggression is measured with the Reactive and Proactive Aggression Questionnaire (REPRO; Dodge & Coie, 1987; Hendrickx, Crombez, Roevers, & Orobio de Castro, 2003). It assesses reactive (3 items; e.g., “If they tease or threaten me, I get angry.”) as well as proactive (3 items; e.g., “If I do not like a child, I will bully himwith others.”) aggression. Students, parents, and (non-trainer) teachers complete this questionnaire. 2