*2.1. Participants*

This study used non-probability incidental or accessibility sampling. The sample comprised 113 students in compulsory secondary education aged 12–15 years (mean age ( *M*) = 13.82 years, standard deviation (*SD*) = 0.79 years). The research was conducted in a state school with students from five class groups. The control group comprised 44 students (two class groups) and the experimental group included 69 students (three class groups). The experimental and control group assignment was based on a cluster-randomised controlled trial. The gender distribution was 64 (57%) boys and 49 (43%) girls (Table 1).

The main inclusion criterion was parental consent. The exclusion criteria were: (a) attending less than 80% of the sessions of the intervention programme (less than 13 sessions); (b) students with special educational needs associated with intellectual disability; and (c) students that were removed from school for disciplinary reasons.


