Social network research has evidenced the role of peer effects in the adoption of behaviours. Little is known, however, about whether policies affect how behaviours are shared in a network. To contribute to this literature, we apply the concept of diffusion centrality to school tobacco policies and adolescent smoking. Diffusion centrality is a measure of centrality which refers to a person's ability to diffuse a given property--in our case, smoking-related behaviours. We hypothesized that stronger school tobacco policies are associated with less diffusion centrality of smoking on school premises and of smoking in general. A whole network study was carried out in 2013 and 2016 among adolescents (n = 18,805) in 38 schools located in six European cities. Overall, diffusion centrality of smoking in general and of smoking on school premises significantly decreased over time. Diffusion centrality of smoking significantly decreased both in schools where the policy strengthened or softened over time, but for diffusion of smoking on school premises, this decrease was only significant in schools where it strengthened. Finally, stronger school tobacco policies were associated with lower diffusion centrality of smoking on school premises and of smoking in general, though to a lesser extent. With such policies, smoking may, therefore, become less prevalent, less popular, and less clustered, thereby lowering the risk of it spreading within networks in, and even outside the school.