A student's performance in school is influenced by personal characteristics, but also, amongst other influences, by those of his or her schoolmates. Schoolmates can motivate and help each other overcome learning difficulties; but they can also disrupt instruction, require disproportionate attention from teachers, and be a source of anxiety. The way students are allocated to schools, and whether that results in greater socio-economic or academic differences across schools, may thus have an impact on education outcomes at the country level. In which PISA-participating countries and economies are students concentrated in certain schools, depending largely on their ability or socio-economic status? How is socio-economic segregation across schools related to the achievement gaps between students of different socio-economic status?