I'm a secondary school teacher, and while I'm horrified (but not hugely surprised, sadly) by what's happened, it's heartening to see that people on Mumsnet have some empathy for the teacher.
I use a teachers' forum and I've just come from there, with some trepidation (I didn't imagine the reactions over here would be so sympathetic) where the event is being discussed at length - the prevailing attitude being, 'There but for the grace of God...'
I've worked in a couple of schools where some of the pupils are off the wall, behaviour-wise. Sadly, we live in a time and society where some children fear nothing - teachers have very little power over pupils anymore. Force of personality is often the only thing teachers have to handle certain volatile situations.
The teacher involved was very well thought of, but had been off for some time with a stress-induced stroke - going back into that kind of environment after an ilness of that nature must have been dreadful. What I'm trying to say, is that dealing with certain types of behaviour from pupils when you're confident, on top of things, and generally in a good place is hard enough. This man was ill, very ill. He shouldn't have been in the classroom.
Saying all this, I obviously hope the lad pulls through. What a dreadful thing to happen to him. What a horrific trauma for ALL involved.