Students in some countries do far better on international achievement tests than students in other countries. Is this all due to differences in what students bring with them to school – socio-economic background, cultural factors, and the like? Or do school systems make a difference? This...

We exploit rules of class formation to identify the causal effect of increasing the number of immigrants in a classroom on natives test scores, keeping class size constant (Pure Composition Effect). We explain why this is a relevant policy parameter although it has been neglected so far. We show...