I went to a rough as arseholes school, and it was weird in the fact that the class could 'sense' a weak teacher, and it was almost an unsaid plan to try and crack the teacher up. There would be a couple of kids who would kick off, and the rest would go along with them in almost passive agreement to try and wind up the teacher.
The same kids in the same class would behave themselves well in a different class, as we could sense the teachers with 'authority', for want of a better word.
Have seen several teachers either walk out of the classroom at the end of their tether, and on one occasion a teacher burst into tears and ran off.
It was a kind of Lord of the Flies mentality, and makes me very uncomfortable to remember it.
I worked at that same school as an adult, the difference was that the disruptive children were removed to an inclusion unit (headed by very skilled staff) and there were TAs in most of the classes. There were still badly behaved kids (like I said, it was a rough area) however they were dealt with, and classes did not descend to the same level of anarchy as I experienced when I was a pupil there (early 90s).
I feel very ambivalent about this case, clearly the teacher was at the end of his tether, however if someone with post traumatic stress and depression had a set to with a kid in the street, and battered the kid around the head with a dumb-bell saying 'die die', he would (I believe) have been convicted. As it was, he was in a school so I think he has got away with it as the jury blamed the feral teenager. Wrong in my view.
He obviously was not fit to teach. He should have recognised that.