I hope you see this before you go to the school today: Do NOT meet with the teacher alone, just you and her, but yes, do meet today -- don't let her or the school put you off and say they'll discuss it sometime next week etc. Be polite and calm but insistent. Say that you will meet with her today and that the principal or vice principal must be present. This puts her on notice that this is a larger issue than her being able to simply rip up a student's test.
Both teachers handled this horribly. The fact that the teacher actually said "he looked guilty" because he didn't speak --? Pure nonsense.
She should have brought BOTH students to the principal and there should have been a talk with not just your son but the girl. And then you should have been called. And she should not have torn up anything -- if there was cheating, she has now destroyed the evidence.
Let's be clear here -- the girl isn't necessarily a villain; she may well have sincerely thought he was cheating, and if your schools are like ours here, the kids have been taught, certainly by about fifth grade, that cheating is absolutely wrong and they must report it even if it means getting another kid in trouble. So don't go in, guns blazing, and accuse the girl of lying; that could only backfire on you.
Do go in -- with a third party like a principal there, because the teacher's boss should know how this was handled -- and ask:
Does the school have a written protocol for how a teacher is supposed to proceed in this situation? Say you need to have them produce that. Sit there and read it while they wait. Did her actions follow that protocol?
Why was this handled DURING a test, when the act of walking in and tearing up a test was clearly going to distract other students badly? They were focusing on this little scene and not on their own tests.
Why were both students not questioned? What made the girl think he was cheating off her?
Why does the teacher believe that your son's saying nothing was an admission of guilt? That wouldn't hold up, well, anywhere. Does she have some other specific reason she would immediately believe he was cheating? Has there been some incident she has not informed you about? If so, she needs to do it right there in front of a principal.
Tell the principal and teacher exactly what your son said about how he is always told not to talk in class and be clear that he feared the repercussions of defending himself even more than he feared her tearing up his test.
Tell them that you know it's "he said, she said" but the school failed to do any questioning of either child beyond assuming your son's guilt because he was too scared to defend himself.
Tell them that humiliating him with an instant assumption of guilt, and a dramatic gesture of ripping up a test in front of the entire class during a test (!), raises the issue of whether the teacher has any experience handling this previously. The teacher should know that assuming guilt is, frankly, opening herself up to parents reporting her to the school --which is what you need to do. Even if your son DID cheat (and I'm assuming here you believe him when he says he didn't), the way it was handled was extremely poor and actually distracted other students, and created a atmosphere where your son now must "live this down" the entire rest of the school year.
I would then tell the teacher and principal, "Now, what do you propose to do about this situation? Because this is not over." If they don't recommend that your son retake the test, then you insist on it. Don't walk out of there with it unresolved. Be clear that both your son and you have lost confidence in the teacher because of how this was handled with assumptions and humiliation rather than calm investigation.
Once a student loses confidence that a teacher will act fairly and calmly, the student has to spend the rest of the year with that lack of confidence and it's crushing to a kid's school experience--every single day. I know, having seen it happen already this year with a friend's fifth grade son. The parents ended up demanding, rightly, that he be moved to another class, and that has been an immense help. If your son's teacher does ANYthing after this that makes you feel she's scapegoating him because you came in and got her in trouble -- say that he must be moved.