What should we do, we're not even on speaking terms?

Me and my best friends had always been really close, but in the past year one of our friends, had become really bitchy and saying horrible things about everyone, and not just things like ‘your ugly’ well we decided to confront her one day as it was getting out of hand, then she told her form tutor we were ganging up on her, when actually half of us weren’t in the room, it was only 2 of us, and we were only going to ask her about it, but then she started crying, and everyone went onto her side except some people. Her mum then told our head of year, and he is doing sh*t all about it, so we have to sort it out by ourselves which doesn’t help seeing as we’re not even on speaking terms now, (for example: today in maths we had a new seating plan and I got moved next to her; normally we would of been chatting like hell, but today we didn’t say a word to each other). The other thing that has annoyed me is the fact that she went off with 2 people christine and tom, christine is also our friend but not best friend, and our old friend always talked about them saying they were idiots, gay and annoying and she hates them, now she’s always with them non-stop and they really don’t deserve to be used by her as they’ve gone through so much crap. Well how could we sort this out when she won’t even talk to us, we don’t wanna be her best friend, just on speaking terms again… ?