In the US in 2010, cost per inmate per year in a state prison ranges between $14,603 (Kentucky) and $60,076 (New York), and averages at $31,286. That's 16 years ago, so it'll be higher now. In the UK it was an average of £32,315 in 2020-21. You might as well employ an individual case worker, and the societal outcomes would be a hell of lot better.
True, but we have institutions dedicated to dealing with people like that.
A school isn't that kind of institution and will fail in its mission (to protect and educate) if it tries to fill the role of controlling violent people.