OBJECTIVE: The aim of this study is to test whether participation in the Smoke-Free Class Competition (SFC), a classroom-based smoking prevention program, reduced smoking and increased smoking-related knowledge among those with and without smoking peers compared to control groups. METHOD: Including smoking peers as a moderator in multilevel models, a quasi-experimental design with a post-test at seven months was used to analyze data from 1035 students in Switzerland (2011; mean age 13.3, 53.2% female). RESULTS: Completing the SFC decreased smoking prevalence (OR=0.7; CI=0.5-1.0; ns) and increased smoking-related knowledge (b=1.0; p<.01) compared to control classes. However, the former effect was only significant among those without smoking peers (OR=0.3; CI=0.2-0.5; p<.001). With smoking peers, smoking prevalence was even higher at post-test (OR=3.7; CI=1.7-8.2; p<.01) in the classes that completed SFC compared to controls. No effect was found in classes that started SFC but had to pull out. CONCLUSIONS: The results indicate that SFC should be applied early in adolescence, when individuals are less likely to have classmates who smoke and are therefore less likely to have to pull out of the program. This is particularly important because SFC was only effective among those who completed the program and did not have smoking peers.