Ethical considerations
The study was conducted in accordance with the Declaration of Helsinki and approved by the Research Ethics Committee of the University of Tartu (code: 327/T-4, 19.10.2020). We used the CONSORT checklist when writing our report [Informed consent forms and study information were distributed by research team members, who also answered all questions that arose. Students were given information about the study, including the purpose, procedures, duration, potential benefits, and risks. Students were informed that participation is voluntary, and their anonymity would be guaranteed. To match the responses to questionnaires of one participant over three time-points, a personal code was generated for each student. The personal code was based on school name abbreviation, class number (“6” for sixth graders; “7” for seventh graders), participant’s initials, and gender (“B” for boys; “G” for girls). All questionnaires in three data collection occasions were marked with the personal code that was introduced to participants when distributing the first questionnaires. Data was transferred to electronic form without the assistant knowing the identity of the participants. The study did not harm the participants neither mentally nor physically, invasive research methods were not used.