Esther Mertens

| 37 Study Protocol control including a ‘catch’ question (Kirby, Petry, & Bickel, 1999; Meyerson, Baumann, Green, 2014; 2017). Students can choose a smaller, immediate reward or a larger, delayed reward (e.g., “Would you prefer to receive €54 today or in 117 days €55?”). To ascertain that the students have read the questions, a catch question is added, similar in form, amount and delay: “Would you prefer to receive €59 today or in 139 days €21?”. Self-reflection will be reported by the students using the Engage in reflection subscale of the Self-Reflection and Insight Scale (SRIS; Sauter, Heyne, Blöte, Van Widenfelt, & Westenberg, 2010). It contains 6 items (e.g., “I don’t often think about my thoughts.”). Self-esteemwill be measured with the subscale Global self-perception of the Self- perception Profile (Harter, 1988) reported by the students. This subscale contains 5 items (e.g., “I’m often disappointed in myself.”). Emotion regulation is measured using the subscales Impulse control (6 items; e.g., “When I’m upset, I feel out of control.”) and Strategies (8 items; e.g., “When I’m upset, I believe that I will remain that way for a long time.”) from the Difficulties in Emotion Regulation Scale (DERS; Anderson, Reilly, Gorrell, Schaumberg, & Anderson, 2016) completed by students. These subscales measure students’ ability to control their emotional impulses and the regulation strategies they apply. Deviant and prosocial communication is assessed using video-observations of same-sex dyads of classmates in a subsample of students in the ‘Standard’ and control condition. This observation task is based on the Peer Interaction Task (Dishion, Andrews, & Crosby, 1995). The dyads plan an activity together, as warm-up, and subsequently discuss 3 situations concerning daily school situations. Each of these 4 segments lasts 5 minutes. The 20 minute interactions are videotaped and coded. Deviant and prosocial communication is coded based on the Conversation Topic Code (Piehler & Dishion, 2004; Van de Bongardt et al., 2017) and communication ratings (Piehler & Dishion, 2004; Dishion et al., 1989; Whalen, Henker, Collings, McAuliffe, & Vaux, 1979). Treatment adherence To assess treatment adherence, the trainer indicates after a series of three lessons which strategies were used, level of treatment adherence, and whether the lessons were completed. Furthermore, a subsample of lessons will be observed by an expert in R&W to assess treatment adherence, quality of delivery, participants’ engagement, and adaptations. This coding schema is based on Bishop and colleagues (Bishop et al., 2014). Treatment adherence indicates the level to which the trainer has conducted the lesson as described in the manual (e.g., “Skipped the trainer exercises?). Quality of delivery is an indication of the general quality of the lesson (e.g., “Are the goals of the lesson met?”). Participants’ engagement indicates the level to which the trainer 2