Numerous studies have shown that an open classroom climate for discussion increases students' civic knowledge. However, most previous studies draw on cross-sectional data and have not been able to show that the effect is causal. This article presents results from a Swedish panel survey following students during the first year in the gymnasium (upper secondary level). Using this study, we are better equipped to evaluate the link between an open classroom climate and political knowledge. Results suggest that the effect is causal. A 10% increase in open classroom climate is associated with about 5 percentage points higher knowledge. The beneficial effect of an open classroom climate is an important insight that should be seriously considered not only by researchers but also by educational policy makers, school managements, and teachers.