Hi givemeagin so sorry you and DD are going through this. She sounds lovely ( and you sound like a great mum) and very like my DD 15yrs. She has gone through very similar exp at school and has not really had any friends at home either so I know how tough it is.
You will prob have already had a good chat about how mean girls can be and how none of this is her fault. Mention the posts her off mums saying all the same thing, girls can be so mean and bitchy. Honestly DD comes home with tales of this every day of constant petty meanness and bitchiness. I tell her it is them the one with the problem, not her. Nobody deserves to be called names, ignored etc.
Somehow she needs to develop some coping mechanisms, I had to follow the same tack as Ledkr (excellent post) and really befriend her myself, listen to her, do stuff with her, be there for her. I told my daughter all the good things about herself, she was pretty, kind, caring etc etc. A good solid relationship with you will be worth it's weight in gold. Have as much fun at home as possible, watch films, DVDs together, play some boardgames, baking, anything .
Clubs have been mentioned lots of times so I won't repeat except to say it did really help my DD to join a martial arts club but I know it's tough if she has low confidence (but reassure her there are bound to be people joining who are in same boat).
And is there anything maybe at the local church/? I don't think it's necessary to be super religious to have a bit of a social life through the church, at ours it isn't necessary.
Hope you find some help and reassurance here
for you