Oh no! A knife! That's so way out of line. How can the school even allow such item to be brought in. I would never allow the teaches or principal to rest until this is resolved.
And NO, you're definitely not over reacting. In fact it's your every right to make this known to the kids parents. Demand to see them and ask what is their plans to educate their son and ensure this does not happen again.
Saying is one thing which is already bad but bringing a knife to achool, that's crossing the line.
I had a child threw a mechanical pencil at my son and the lead went into his ear,a hole was caused because of the incident. I went straight to school the next day and demand the principal to handle this if not I will!
So what if the boy's mom works the school. She needs to be fully responsible for her son and not hide behind the rules of the management.