Loss of the use of corporal punishment as a deterrent to bad behavior. Judiciously applied, corporal punishment is a great extrinsic motivator to keep a kid from disobeying. Not saying we should hit kids, just that there is no mechanism where you can break down their resistance. Bullies do it all the time to control their peers. All teachers can do is ask, request, plead, argue, debate, convince, cajole, beg, deal, negotiate, threaten, but in the end if the kid doesn’t want to go along or to get along, the effort is wasted, and a great deal of time is consumed. I laugh at liberals who don’t believe in corporal punishment, they sowed and now they reap.
Another thing about teachers, many of them do not have children of their own. You wouldn’t believe how many teachers don’t have any kids, but they know better than you how kids should be raised.
At times a device like the agonizer from old Star Trek would be great for schools.