I don't get the expulsion discussion here (not just your comment, but responses as well). At least in the US, these kids legally need to be educated somewhere. Expelling doesn't remove them from the school system. It just means they'll be disrupting a different set of kids at some other school. Making something someone else's problem isn't solving the problem. It's like dealing with homelessness by rounding them up and bussing them to a different city.
You could conceivably create schools composed entirely of the kids who got expelled from somewhere else, which I think some districts have actually done (continuation schools), but you're effectively creating a juvenile prison system for people who didn't commit any actual crime and definitely never got a trial.
I think that's pretty much the crux of every argument here - move those other kids somewhere else. Don't worry about improving their situation, but make my situation better.
Cite a study, draw incorrect conclusions from it that support your argument, find some exception to the norm and use that as your example, and don't forget - describe how you were brought up and how that was better.
You could conceivably create schools composed entirely of the kids who got expelled from somewhere else, which I think some districts have actually done (continuation schools), but you're effectively creating a juvenile prison system for people who didn't commit any actual crime and definitely never got a trial.