As already mentioned, I would report this incident to the police and ask them what they can do about it, seeing as the school doesn't want to broach the subject to intensly. Stabbing your son, whether it be a pen or a knife is assault (maybe even aggravated by the letter of the law) and as such should be dealt with in the confines of the law!
Perhaps you may wish to discuss your intentions with the school before you go ahead and make this decision, they may actually pay heed to what you are saying then
Lets hope your son can carry on as normal without the need to be uprooted from his friends that he has already
