Doing so leaves teachers in the position of a paper tiger with rules they must enforce, but no means to enforce them. In the long run, everyone suffers the classmates of the misbehaving students, the educational process, society in general, and the misbehaving students especially. Unfortunately, there will always be kids who will not behave in an acceptable manner without a little coercion. Teachers must have some means of curbing this behavior. The current trend is a series of warnings that culminates in expulsion.
Some school systems expel kids from the regular school to a special school for problem kids.
More bad behavior gets them kicked out of the special school and onto the streets. In the meantime, the entire educational system suffers. Classes are disrupted, teachers are distracted, authority becomes a joke, and well-behaved students have to put up with the unpunished behavior of a few bad actors. Eventually the kid may get kicked out of school. Now we have a juvenile running loose with nothing to do and probably no parental supervision during the day.