While the current empirical literature on peer group effects in schools highlights that credible causal peer effects cannot be estimated unless parental sorting is taken into account, the present paper highlights that causal peer effects might be conditional on the learning environment in which...

We report results from a large-scale, pre-registered randomized field experiment in 159 Norwegian schools over four years. The intervention includes students aged 7-9 and consists of pulling students from their regular mathematics classes into small, homogenous groups for mathematics instruction...