A highly publicized study published in Nature on the impact of schooling on students after adjusting for genetically heritable traits finds:
“There are … substantial mean differences in [the] performance between pupils in selective and non-selective school types. However, almost all of these differences are explained by heritable, individual-level factors, which schools actively or passively use in the pupil selection process.”
The study concludes:
“We are not saying schools are unimportant, or that teaching does not work. Without schools, it is hard to imagine a successful education system that allows children to reach their academic potential. However, while schools themselves are important for academic achievement, the type of school appears less so.”
Perhaps non-selective schools also sort students.