*2.3. Procedures*

All materials and procedures were approved by the data protection officer and educational authority of the federal state of Bremen, Germany, as well as University Institutional Review Board. Twenty schools were randomly selected from a list of 167 schools. From these 20 schools nine principals did not reply to the recruitment email, four expressed interest but had existing commitments that prevented them from participating, and seven provided agreemen<sup>t</sup> to have their school participate. There were 1788 parental permission slips passed out to the students. Of these, 1480 parents/guardians agreed to allow their child to participate. Reasons for not participating in this study were missing written parental consent, sick note, absence because of projects, internship, refusal to participate, unexcused absence at school, being new to the class and therefore not informed about the survey, or having refugee status (missing German language skills). An online survey was conducted during one regular school hour in the school's computer lab. All participants received instructions and were informed that their participation was optional, that they could choose not to answer questions, and that participation could be stopped at any time without giving a reason and with no consequences. In order to prevent distress and further harm by participating in this study, oral and written information where

those who had taken part in the research could ge<sup>t</sup> counseling online and offline was given. Less than 3% of the data was incomplete and the missing data were handled with mean imputation [25].
