1.) Show up in person, tomorrow. You don't have to be aggressive, or hostile. Don't worry. My experience with administration is this: they are human. Well, by and large, they are liberals, but at least in terms of DNA, they are human. And frankly, they are always happy to know that a child has a concerned parent or two.
2.) Be sure you know exactly what your concerns are when you show up. Are you concerned that
a.) Your child was traumatized?
b.) Given a message you don't agree with? (knives are bad, telling adults is always good, what?)
c.) This will go on his "permanent record" (I wouldn't worry too much about this).
3.) Don't assume the school is out to "get" your child, on the other hand, don't let them intimidate you. Assume you both want what is best for your child (they really do want what's best, their error is usually in the "what is best" realm. They have good intentions.)
So what I'm saying is go in there with guns loaded, but don't shoot first and ask questions later. Know exactly what you're worried about, know exactly what outcome you hope for, and go in knowing that they mean well, but you might have to gently but firmly set them straight.
Sonar, wizardoz gives the best advice on the thread so far, and I've already made 2 comments! Although I must say, don't listen to the advice contained in his tagline!