were invited to participate in the study. The schools were selected based on their size: each school had at least 100 pupils in the seventh grade. Letters explaining the study protocol were sent to the chief education officers of the cities and to the head teachers of the schools, and their approval to participate in the study was sought. In one city, all six schools refused to participate mainly because
they had already participated in many projects and studies. Finally, twelve schools in three cities (Tampere, Lahti and Mikkeli) agreed to participate.