What interfered with that balance?
But the big thing, since you asked, is something passed in 1965 (part of the Great Society) called Title I which among many other things governs Special Ed kids. As time has gone by, more and more things have become “special”—including behavior disorders. As soon as a kid is given an I.E.P. (Individualized Education Program, i.e. officially “special”) a school has to jump through several hoops to impose any punishment at all for virtually any misbehavior, including assault on a teacher.
Parents of special kids basically have a school district by the short hairs. Most don’t know it, but those who do can make life miserable for teachers, staff and administrators. Even small districts will have on staff someone who makes sure all t’s are crossed and i’s dotted—it is a very big deal. As a bonus parents of ‘special’ students can qualify for SSI from Social Security. This is called ‘crazy money’ in some places.
So things that worked, or at least somewhat worked to keep discipline in schools have been taken away and pretty much replaced by nothing. If a school or district is lucky and has students that don’t constantly act out, then things are pretty good. But if the percentage of ‘feral’ children gets too high, education for all but the most strong-willed is nearly impossible.