In the US it used to be more acceptable to put students in classes based on where they were academically and in terms of behavior. Truly disruptive kids were also usually suspended until eventually being expelled.

Neither approach is considered appropriate anymore. It was a central thesis of No Child Left Behind, which is/was nice in theory, but not in practice.

Now schools are data obsessed. Bad metrics like suspensions, expulsions, etc are avoided because suspensions are correlated to bad test scores and bad future outcomes for students. So schools now measure performance against that. The problem is obviously correlation vs causation, but if that is how you are graded as a principal that is what you are going to work towards to keep your job.

The reality is that if you want good schools you need to cut your losses with the worst behaving kids. Slow learners really aren’t much of an issue because they don’t disrupt other kids learning beyond maybe needing more teacher attention.