I'm really shocked by these stories. This is a bit tangential but why are pupils who behave like this allowed to get in the way of the process of education?
I think there should be two sorts of schools - not divided by ability, but by behaviour. If you are a student who can behave at school, show other people respect and want to learn, you can be at a normal school and learn stuff with normal teachers in a calm environment. Anyone who demonstrated that they couldn't behave would be moved to an intensive remedial school with security guards to protect specially trained teachers. They would get a lot of help with their behaviour, as well as lessons. If they improved and showed a willingness to learn, they could return to the normal school, but if they caused a problem they'd just go back again.
Yes it would be bloody expensive, I realise that. But, besides the horror of all this for the teachers, what about the kids who aren't causing a problem who have to sit through this all the time when they are supposed to be getting an education?
As has been said most children/teenagers are actually OK. Take the disruptive ones out of the equation and they wouldn't have an influence on the others either.
Re this case, I'm sorry for both the teacher and the student, who may not have been disruptive at all himself - we don't know.