Bearing in mind that the OP has said zip about what they are actually referring to :
The first answer is limiting isolation, 1 day, 2 days... Fair enough, days and weeks and months? No fuck that That points to massive failures on the schools and teachers part to deal with behaviour issues.
Possibly
Why aren't they telling the parents?
Schools never do that because they are bastards, and not bound by law.
Why aren't they trying different methods of discipline?
They can't be arsed
You beat a child once, they learn to fear, they don't learn to behave
They can't hit kids now. Do keep up.
Put a child in isolation repeatedly day in and day out, what's that teaching them?
It's not beating. Possibly at the least they aren't stopping others learning.
Take them out of the class they're disrupting, get them a five and doing something, cleaning the school, sweeping the school hall, working in reception, helping admin with menial tasks etc etc etc.
Never been in school where this is allowed. The pupil must be given work related to education.