Background: An adolescent’s perceptions of their family’s and friends’ smoking attitudes and behaviour can influence their own uptake of smoking. There are two broad sources of such social influence: observing the behaviour directly, and assimilating attitudes. Methods: We analysed data collected for the evaluation of Dead Cool, a school based smoking prevention intervention in Northern Ireland (n = 480 in 20 clusters). The main analysis fits three nested logistic regression models predicting pre-intervention susceptibility to taking up smoking, as reflected in responses to three attitudinal questions. Model 1 includes only personal characteristics as explanatory factors. Model 2 adds the behaviour of friends and family that would provide an opportunity for social influence through observational learning. Model 3 adds the susceptibility of friends. Results: Each additional group of variables improved the model fit (with reduced AIC and BIC). However, in the final model, only three variables were found to be statistically significant (p < 0.05) in predicting susceptibility to smoking initiation: rebelliousness (OR [ 1.1, 1.3]) from the personal characteristics group; and, in the observational learning group, being friends with a smoker (OR [ 1.0, 2.9]) and frequency of being in the same room or car with someone smoking (OR [ 2.0, 9.0] for most frequent). Adding the two measures of diffusion of susceptibility through the friendship network improved the model fit, but neither was found to be statistically significant. Conclusions: The analysis provides additional evidence to support policies that could reduce children’s exposure to smoking behaviour, and potential subsequent smoking initiation. No conclusions could be drawn about the diffusion of smoking attitudes through the school friendship networks of children.