AS if anybody cared - how it works in Russia: - There are virtually no private schools, every school is public. - There are better schools and worse schools. In a big city there are a few really good ones and a bunch of mediocre ones. - A school have to accept anyone living in a proximity, but it may also accept anyone else. So the good ones set up entry exams. Mediocre ones enroll everybody. - I guess there are some…
Same process in Poland (start by enrolling anywhere, later exams from the previous school matter), yet the difference was big. High schools in my home town got between 98% and ~10% people advancing higher. I guess 98% wasn't 100 because kids moving to schools abroad were counted as "not on the list of local universities". For primary school you pretty much started in a mixed, local group, without any visible bias, so…
Those who have the motivation and background to continue to higher education also tend to shift towards "good" schools. They would continue even if they didn't go to a "good" school.