Public schools require the participation of parents in order to maintain discipline and to actually function as an institution to educate our children. I can't believe people are actually recommending you retain counsel to resolve this matter. If your son doesn't realize that a pocket knife is considered a weapon in today's society then you should educate him. Then you should make sure he knows the rules of the school he's attending and discipline him yourself when he breaks a rule.
My dad was a teacher for 30+ years and at the end of his career had a parent file an assault suit against him because he had to physically remove a kid from a climbing rope after he'd told the youngster (elementary school) to stay off the rope several times and the boy refused. A court case and lawyers fees because a parent couldn't face the fact that little johnny couldn't follow the rules.
Slap yourself and sign the form.
So reasonable :~D