Schoolchildren aged 12-13 years in the first year of Compulsory Secondary Education (Enseñanza Secundaria Obligatoria in the Spanish educational system) in the city of Terrassa. The CONTROL arm is formed by schoolchildren in schools that willnot receive the intervention and will continue with ongoing (usual) health education sessions.

The intervention at the classroom consisted of six sessions with the pupils of one hour each that were conducted by the teacher/tutor. We provided the teachers with a training session and a teachers' guide, and we gave each pupil a workbook with the activities. At the school level, the intervention consisted of four types of posters with specific messages directed to students, teachers, and parents, and the fourth poster type advertised the new smoking laws. we gave teachers and school managers the guide "Towards a Smoke-Free School" to facilitate the prevention and control of smoking (active and passive) in the school environment. Family level activities: Parents were required to complete the "My risk thermometer" activity at home with their children. Parents received a brochure with information on the risks of SHS exposure and recommendations to prevent SHS exposure, and a refrigerator magnet with the logo of the program.

The intervention at the classroom consisted of six sessions with the pupils of one hour each that were conducted by the teacher/tutor. We provided the teachers with a training session and a teachers' guide, and we gave each pupil a workbook with the activities. At the school level, the intervention consisted of four types of posters with specific messages directed to students, teachers, and parents, and the fourth poster type advertised the new smoking laws. we gave teachers and school managers the guide "Towards a Smoke-Free School" to facilitate the prevention and control of smoking (active and passive) in the school environment. Family level activities: Parents were required to complete the "My risk thermometer" activity at home with their children. Parents received a brochure with information on the risks of SHS exposure and recommendations to prevent SHS exposure, and a refrigerator magnet with the logo of the program.

The prevalence of exposure to secondhand smoke is computed by dividing the number of children exposed / total number of children. Exposure to secondhand smoke is obtained using a questionnaire at baseline and one year later. The questionnaire was prepared from previously validated questionnaires on SHS.

The prevalence of smoking is computed by dividing the number of smoking children / total number of children. Smoker status is obtained using a questionnaire at baseline and one year later. The questionnaire has been previously validated in other investigations.