The lesson for school librarians today is...
When a student with behavioural issues tells you that if you tell them to get off the computer they will "punch you in the balls".
That said, this sort of behaviour is noting new for a librarian. I remember a homeless man pulling a knife on one of the security guards one night when were were kicking everyone out of the state library.
I also remember a tiny little woman walking into Alice Springs library holding a large branch, walking up behind her husband (who was watching a video) and belting him across the head. When I ran over to stop her going for a second swing she looked at me and said "but he hits me" in such a sweet old lady voice.
Or, there was a patron once who refused to get off the computer (a backpacker) so I flicked the power switch off with my toe and guided him (bitching and moaning) to the exit with my hand on his back giving him a gentle shove. 5 minutes later a police officer turned up to investigate the report of a librarian assaulting a tourist. (at this point all the ladies in the large print section came over and explained to the officer that the nice librarian was; 1 a librarian and 2 nice, while the tourist was neither.)
Looking back on the library jobs I have had...
I do note that I did not see any violence during my 2 years in a theological library, nor during the 2 years I worked in cataloguing.
But the school seems to have better policies in place to deal with these things than any other library I have worked in. Not only did I know who to report the incident to, but I was kept informed about what they were doing/saying. Yep I think I like that about schools (well, that and students with behavioural issues tend to be smaller and less psychotic than the homeless).