Hi B.,
Here is my advice.. You should NOT trust that the school is handling it. In my experience schools only handle issues like this when you make it known to them that you are taking this serious.
Step 1: Get into the school asap don't call first, just show up and start with the guidance counselor explain everything, including how you feel that the school is not handling the issue. Let him/her know you will not tolerate this and it's their job to keep your kid safe even from verbal threats.
Step 2: Call the boys parents, talk, see what your vibe is. Start with something easy, "hi, this is --- mom, i just thought you would want to know that --- threatened to bring a knife into school, i don't know if this is a normal behavior for him but i wanted to let you know about it" this way you are not "attacking" their child. Explain the situation and let the other parent know that you will not tolerate this, and that you have spoken to the guidance counselor already. It is tricky if you don't know them and usually behaviors like this start at home so dealing with the parents is not always easy, but it also could be fine - you never know.
Also, if your son was saying something like this and another parent knew wouldn't you want them to call you? To let you know? maybe they have no idea.
Step 3: Follow-up with the school, with your son - have they done anything, has the child stopped etc., etc.,
And remember that this kid might have just been having a bad day, or maybe somebody threatened him first. You don't know the entire situation only what you heard from your son's point of view - so keep an open mind. Also this kid might be serious, he might really bring a knife to school sounds far-fetched but nowadays it's not - so you can't let it go. Make the school hear you. Good luck!