Background:
The school environment is an equitable avenue to change youth health behaviours; however, there is little guidance for school-based programming. Specifically, the types of interventions schools use to prevent smoking uptake and progression are unknown. This analysis explored changes in school-based tobacco control programs and policies and the impact on smoking status.

Methods:
COMPASS is a prospective cohort study that collects hierarchical longitudinal data from students and the schools they attend in Canada. Two cohorts of grade 12 students from 38 schools were identified at baseline (Y1) and three years later (Y4). The prevalence of susceptibility to future smoking, ever smoking, and current smoking among cohorts were compared in Y1 and Y4 in control (no change in tobacco control programing or policies) and intervention schools (reported a change in tobacco control programming or policies at any time during the study).

Results:
Between Y1 and Y4, 33/38 schools reported changing their tobacco control programming or policies; 19 schools made a single change over the study period. Changes to enforcement activities and cessation programming were most common and usually involved external support. Few interventions significantly impacted the prevalence of susceptible never-smokers or ever smokers. The prevalence of grade 12 current smokers significantly increased at 12 schools and decreased at 3 schools. Thirteen interventions resulted in significant increases in students' perceptions of support to resist or quit tobacco.

Conclusions:
Many schools implemented new tobacco control programming over three years with support from external organizations; however, few interventions significantly improved the smoking status of students over the long-term. Nevertheless, schools increased awareness of the issue of smoking by making a change to tobacco control programs or policies, which may support the behaviour of future students. The COMPASS study provides the infrastructure to continually monitor and evaluate school-based activities designed to advance youth health.