The purpose of this study was to comprehensively assess the impact of school tobacco policy intention, implementation and students' perceptions of policy enforcement on smoking rates and location of tobacco use during the school day. Data were obtained from all students in Grades 10-11 (n = 22,318) in 81 randomly selected schools from five Canadian provinces. Policy intention was assessed by coding written school tobacco policies. School administrators most familiar with the tobacco policy completed a survey to assess policy implementation. Results revealed policy intention and implementation subscales did not significantly predict school smoking prevalence but resulted in moderate prediction of tobacco use on school property (R(2) = 0.21-0.27). Students' perceptions of policy enforcement significantly predicted school smoking prevalence (R(2) = 0.36) and location of tobacco use (R(2) = 0.23-0.63). The research findings emphasize: (i) the need to consider both written policy intention and actual policy implementation and (ii) the existence of a policy is not effective in controlling tobacco use unless the policy is implemented and is perceived to be strongly enforced.