Previous research has shown that ethnic school composition can have an impact on a number of outcomes for pupils. The influence ethnic school composition has on pupils’ religiosity, however, has not received much attention. Furthermore, the few previous studies that have examined this relationship have relied on cross‐sectional data, thus being unable to separate selection effects from causal effects. In this research note we use longitudinal data collected among pupils in the third (2011–2012) and sixth grade (2014–2015) of secondary schools in Flanders, the northern part of Belgium. We examine changes in religiosity among pupils using cross‐classified multilevel analyses. The results show that there is a positive impact of ethnic school composition on ethnic minority religiosity. This does not mean, however, that pupils become more religious in schools with a higher share of ethnic minority pupils. We rather see that a decline in religiosity among highly religious pupils is attenuated in schools with more ethnic minority pupils.