Willbreakmybones, I think you'd find that quarantining sufficient students in PRUs & special schools would be problematic, as an answer to 'hard to reach' students.
I also think that your suggestions are quite breathtakingly offensive to the parents of some of those who 'take up so much teacher time and energy'.
I teach in a good comp, & there's maybe 1% of our students who really do struggle to access the mainstream curriculum, or to cope with its demands on their behaviour.
If you wanted to re-create a private school environment, OTOH, you'd have to remove 5% at least. Overwhelmingly, their problem is that neither they, nor their families, value education or enjoy school.
It's a problem.
But I've said it before - if you want to make a school successful, make it difficult to get in. You can make the parental requirement money, membership of a religious group, willingness to enthusiastically counterfeit membership of a religious group, or willingness to stand naked on one's head in the town square for half an hour every Sunday for a year or two.
Any of them will work - just make sure you've got parents who've made some sort of investment & will be on their kids' backs to recoup it. Oh, & you need to be able to kick them out - to become another school's problem - as a secondary strategy.
Sorted.