A randomised controlled trial with two parallel groups is performed, with 1:1 allocation ratio, in two different Regions of Italy (Piedmont and Veneto). Schools are randomly assigned to one of the two groups (DDS-2 intervention vs. no intervention) and are used as the unit of allocation. Individual participants (i.e. students) are the unit of analysis. Schools are matched by number of students and average socioeconomic status. Eligible schools are identified from the list of all schools having students from 11 to 13 located in study sites. A number of eligible schools accounting for the requested number of students in the target grades is invited to participate in the study. If a school does not respond to the invitation or declines to participate, the school recruitment process is repeated, and another eligible school is invited to participate. The process continues until achievement of the sample size. No incentive is offered to students, families and schools for participating in the study.