The increasingly popular concept of polarization is used to describe various social phenomena such as ethnic, political, and income polarization. Scholars study ethnic polarization because they assume that it is linked to conflict. So far this relationship has been investigated relying on cross-country data. Evidence is mixed but suggests that bipolarity of equally-sized ethnic groups (polarization) is the most conductive scenario for conflict. While current research concentrates on the macro level, we argue that the assumed causal link is best studied in the setting of small groups. Consequently, we study the link on the level of school classes and analyze data from the Children of Immigrants Longitudinal Survey in Four European Countries (CILS4EU). Our sample contains around 800 high school classes located in the Netherlands, Germany, England, and Sweden and we explore how the ethnic composition of school classes relates to intergroup conflict. We construct a set of treated - ethnically polarized - and a set of - non-polarized - control units and estimate the average treatment effect on the treated (ATT). Our preliminary findings suggest that there is a causal effect of ethnic polarization on the prevalence of conflict among classmates. The results also suggest that it is ethnic polarization, rather than heterogeneity per se, that is the driving force behind intergroup conflict.