From the perspective of a teacher, it is very difficult to monitor all of the children in a schoolyard. There are so many of them, and oftentimes, yard duty areas are quite large. However it is not alright that your son has been subjected to this. I would've thought that bringing a knife to school and threatening other students would result in, at the very least, a weeks suspension, followed by some very stern warnings of lines that will not be crossed lest expulsion be bought upon this child very quickly.

You are to be commended on your patience in this matter. Had my son been in the position of your son, I would have been a writhing mess of absolute fury.

You are probably in a position now, where you could quite easily ask the school to do something immediately, or you will be forced to maybe go to the police. You wouldn't be necessarily laying charges, they would just investigate your complaint and follow up if anything was required.

A messy situation that I really hope gets sorted out fast.