It sounds as if the teacher should not have been teaching that class alone, and the behaviour of the kids was disgusting. I am not without sympathy for the boy injured, he will carry the scars forever.
I think as a society we are nasty and mean. You can see it here on Mumsnet, some posters love to put others down, be cliquey, etc.
In this country we have weird attitudes - childhood obesity, for one. "They'll get bullied!" people say, which isn't inevitable, if everyone bothered to teach their kids not to be nasty to one another it would be purely a health issue! The bullying associated with red hair is another example, it just should be UNACCEPTABLE.
With the election campaign at the moment you can see it, nobody wants to do anything to help anyone else. Nobody is willing to pay a bit more tax. Nobody wants anybody to have anything unless they have more.
When I was in the third year at high school we had an art teacher who was driven to a breakdown. It was awful, I found it painful to watch. I knew the ringleader very well, went all through school with him, lived on the same estate and our mothers were friendly. He tried the same sort of trick on many teachers. His mother NEVER believed that he was capable of such behaviour, but he was probably the nastiest bully I have ever encountered. And he may have been only 14 but he knew what he was doing, and he didn't care.