So in my country Romania, we have national exams, then the children will fill a list of high-schools+ profile pair(HighScoolA - Informatics, HighSchoolB -Informatics, HighSchyoolA -Science ....) so mostly you get in same class children that had very similar results at the national exam.

But there still is the children learn at different speed or some are left behind, at least in my son case I feel in Math they move too fast, there was the same when I was in high school too.