This paper shows how a public policy shapes convergence of beliefs through newly-formed social networks, with a focus on political opinion. We use a unique natural experiment that randomly assigns students into first-year groups at a French college that forms future top politicians. Pairs of students in the same group are much more likely to become friends. The randomized group membership serves as instrumental variable in a dyadic regression of differences in beliefs on friendship. We find that students’ political opinions converge particularly strongly between friends, reaching 11% of a standard deviation only after 6 months. Convergence is strongest among pairs least likely to become friends without the randomized exposure, or friends whose characteristics are the most different. While there is evidence of homophily in network formation, it does not seem to affect the estimates of convergence, except among very similar friends. The same strategy shows that a longer network distance implies slower convergence.