Teachers unions retain a stranglehold
And that is a HUGE part of the problem. Another part is that too many parents expect the school to raise their children for them. And if little Johnny is disciplined they cry that the teacher must not like their child instead of accepting responsibility themselves.
The solution is to teach students who aren't academically oriented trades and life skills along with enough academics to function independently and successfully. Going back to Shop, HomeEc, Hands-On Skills, Budget Management and Small Business Development would also cut down on discipline issues.