Using a unique and very rich PISA dataset from Denmark, we show that the immigrant concentration in the school influences reading and math skills for both immigrant children and native children. Overall, children in schools with a high immigrant concentration score lower on reading and math test scores. The negative effects associated with attending a school with a high immigrant concentration are fairly robust across estimation methods. IV estimates, taking into consideration that parental sorting across neighborhoods might bias the OLS estimates, indicate that immigrant concentration in schools is still important in determining children's math test scores. The estimates are less precise regarding the effect of immigrant concentration on reading test scores. The immigrant concentration in the school has a stronger effect for native children than for immigrant children, but the differences are more pronounced for the math test. (Contains 7 tables and 2 figures.)