Setting and participants
The VET-system comprises a basic program, in two parts, followed by a main program. Each part of the basic program has a duration of 20 weeks (i.e., one semester), with the first part of the basic course aimed at students who left compulsory school since maximum two years, while the second part includes everyone and is an introduction to the main program. VET is divided into four main areas: (1) social and healthcare, (2) commercial, (3) agriculture, and (4) technical [Schools were recruited in two rounds, from January 2018 to April 2018 and January 2019 to April 2019. These time frames were selected to ensure that schools had time to implement the Focus intervention at school start in August. In the first round, seven VET schools were enrolled and randomly assigned to the intervention group (n = 4) or control group (n = 3). It was difficult to recruit VET schools with ‘technology, construction, and transportation’ courses. Therefore, in round two, PBE schools were also included. A total of three VET schools and four PBE schools were enrolled and randomly assigned to the intervention group (n = 4; two PBE schools and two VET schools) or control group (n = 3; two PBE schools and one VET school). One PBE school withdrew their participation and was replaced.The five inclusion criteria were (a) VET school offering education within social and healthcare, commercial, or technical subject areas or PBE school, (b) at least 100 students enrolled in the basic program (first school year) at VET schools or in total at PBE schools, (c) no school-level smoke policies in the form of smoke-free school hours, (d) school management was willing to implement the Focus intervention, and (e) accept the randomization procedures of the intervention. The VET schools were randomly selected by identifying schools fulfilling the inclusion criteria based on data from the student registry whereafter we placed them in random order regarding the sequence of contacting the schools. Due to time constraints, the PBE schools were selected based upon convenience sampling. This involved that we contacted the Danish association for PBE schools as well as a manager of a PBE school who had already implemented smoke-free school hours. We asked these central stakeholders to help point in the direction of schools who could be interested in participating in the intervention.At the VET intervention schools, data collection was performed in the basic course classes where the class-based intervention components were implemented. At the VET control schools, data collection was also performed in the basic course classes. Data collection was performed among all students in PBE schools as the intervention was implemented in all classes.