It's been about 12 years since the last time I worked at it, but I would tend to think the biggest changes happened since then because it's just been in the past ten years that society has undergone such drastic changes.
But I know that even as far back as the early 00's, if a teacher was having problems with a particular student, they had to be really careful and follow these strict guidelines regarding x number of verbal warnings before moving to the next level of x number of written warnings, then x number of written discipline referrals, then written contact with the parents/guardian, then after that, an in person, face to face meeting, etc, etc, all before the school administrators got involved.
And of course, as far back as when I was in school teachers couldn't personally insult a student in front of the class, etc.
As for the old school gruff athletics coach, I knew this one guy I used to sub for or sometimes one of the other coaches and he'd be there, who used to jokingly call the kids "Dumass" (do-MAHSS), as in.....
That's about as close to insulting or name calling as I've seen any teacher dare get.