Zero tolerance also means zero opportunity for the minor to improve and learn. School is also for learning how to function in society, not simply to study the curriculum.

Not to mention that such sanctions end up being applied in a very selective and classist way. Try to send a kid from an affluent family to reform school, and you will have their parents, their parents' friends, the local council, and the local MP on your neck, trying to sway you. But it's "rules are rules" for the kid from the single-parent family who works 10h to make ends meet.