Grouping by ability in one or a few subjects is the most flexible form of selection. In fact, in systems with more academically selective schools, the impact of the socio-economic status of students and schools on student performance is stronger (OECD, 2013a, Table IV. Results from PISA also show that in most OECD countries, students' attainment is significantly lower in schools where most of the students come from disadvantaged backgrounds.