I would have to say that the issue with the school wasn't where the problem started, it was the point where Matt finally broke. He realized that his only option left was drastic and immediate action. I think this is important to remember.
I agree that you need to explain to your daughter that Matt & Si are upset with each other and they have to be separated or they will fight and make each other sad. What has the counselor said on this?
Being stuck in the middle of two people you care about who are at odds and being incredibly stubborn is difficult. The person in the wrong would rather justify their actions, especially if they see the fall out as unintentional
. While the offended party refuses to even listen to an apology even if one was offered, then gets pissed because there was no apology.
Stupid vicious circle, especially when your the one in the middle and you can see where both parties are being a bit unreasonable and essentially shooting them selves in the foot. Good Luck!