Y7 - certainly the first half term, it did get better after that (largely due to the arrival of a new girl who became my best friend and all of a sudden we were the popular ones who the others wanted to befriend) - was definitely hellish for me.
I remember many a break time wandering around on my own or playing British Bulldogs with the boys. And as I was at an international school that went all the way through from Kindergarten to Y13 with the same groupings, it was all the more bizarre as the exact same girls had been in my class for Y6 and we all got on fine then. Equally, when we moved into the upper school for Y8, things remained calm - possibly as we had got the awful hormonal year under our belts and were now the runts of the school, rather than the kings of the middle school.
If it's any consolation, it was just the one half term of hardship and then things settled down - I think my mum just let me get on with it, we didn't involve teachers or anything, though we knew there was a new girl starting and I think the teacher had spotted I was having some ishoos and made me her buddy (one of the benefits of the transience of international schools - possibly not the same at schools here).
My teacher also put me in a different 'activity' from the girls in my class - I did library and they all did pottery or something - on Thursday afternoons, presumably to get me away from an environment where the ostracism would be more noticeable in an unstructured class.
Just one of those things you have to ride out, as others have said, lots of listening to woes, sympathising and maybe try to find something outside school (I had guides, gymnastics, church, swimming and tennis) with completely different kids to socialise with at the weekends if needed - plus LOADS of shopping trips and ice creams. Good luck - sending a 
your way, if that's your thing!