The door for students to assault teachers was kicked wide open the day they removed the sanction of corporal punishment from the teachers' armoury. When I was at school (in the UK in the '50s and '60s) assaults by students on teachers weren't just unknown, they were absolutely unimaginable. The reason? At least six strokes of the cane (or the tawse if you were in Scotland), administered by the head-teacher, in front of the whole school if the offence was considered sufficiently serious for a very vivid warning to be given to the rest of the school. Students knew precisely where the line was, and they dare not cross it - the pain and humiliation were too high a price to pay. It's called discipline.

I know you PC-types will shout me down and throw all the psycho-horse-manure around but, trust me, it seriously concentrated the minds of would-be young thugs as to why they shouldn't do it.
S:0)