This article examines the effect of the ethnic composition in the school class on school performance in primary education, using COOL 2008 data for The Netherlands. We make an important distinction between the proportion of migrant children and the diversity with regard to the different ethnic groups in a school class. Due to the strong correlation between these 2 variables, we employ a residualized score of diversity on the proportion of migrants. The diversity indicator, which indicates the level of diversity given a particular share of migrant children, is negatively related to reading comprehension in Grade 8. For other grade years, we find little support for negative effects of diversity net of the share of migrants in a class.