The gap in achievement between schools at either end of the socio-economic school composition spectrum is substantial in highly differentiated educational systems; there is a steep slope in achievement depending on the socio-economic composition of the school. There is a relatively narrow gap in achievement between the bottom and top 25% of individual socio-economic background (ESCS) in highly differentiated systems. It is a combination of school composition and being “selected” that are the more important factors in determining achievement in these systems