I worked in classrooms with very difficult children. What was the most frustrating was that usually by the end of the week the kids were starting to behave better because they had a full 5 days of being at school and spending less time at home. Then the weekend would come and they spent the weekend out of school and exclusively with their parents and they would be back to square one on Monday. Things would get even worse if there was an extended holiday.
If those kids could be completely taken out of their environment or their parents made some changes, its very possible the kids could change their behavior. The problem is that the kids will normally go back to the same living environment so it feels like all the effort put into trying to change their behavior is a waste of time. It's like Sisyphus rolling the boulder up the hill only to have it roll down again.
Unless you can change the child's home environment, its extremely difficult to change their behavior.
You were a teacher? Which province or city did you work? Public schools?