Backforgood and Justdance, your posts sadly show an incredible lack of understanding - OP, let me regail you with my experience which I think you'll find frighteningly similar.
DD in Yr 7 and 8 had wide group of friends and one full on BFF to use the modern vernacular. Then nasty girl joined this group (manipulative, not very pleasant). Had the power to say who was in and who was out of the group. My DD was out. Lost her BFF, lost the group, lost all groups.
Then had a good solid friendship with another girl, she went off to another group and my DD wasn't invited to so to speak. Repeat once again just in recent weeks.
My DD too is lively, sociable, whacky, outgoing, loves to laugh, and is a very good and thoughtful friend. For all your casting aspertions on OPs DD - you simply don't understand that the power of 'the group' is everything. EVERYTHING. Other kids are so afraid of being picked on to be the one that gets ousted that whilst they might be nice to my DD on a 1:1 basis, when they are in that group they bareley speak to our acknowledge my DD - so in effect, two-faced and unwilling to stand up for what is right and how people should actually be treated.
My DD refuses to sit with them because 1. they make her feel like inconsequential shit, a no one, not included, of no value and 2. because they (as a group) are catty, bitchy little madams and she doesn't want to mix in a group of people like that - why should she especially when it ruins what little shreds of self esteem she's got left? She's exercising a mature evaluation of the situation and saying no that's not acceptable to me and fair play to her for that.
The upshot though is she's currently spending her breaks and lunches in a toilet cubicle because she has no one to hang with or walk round school with or eat her lunch with or go to the cafeteria with.
I totally feel for your DD OP, I'm in the same situation. It's almost impossible to help (or that's what I've found). My DD too because of a variety of family stuff has had to mature a lot and I think this is partly why she finds all this 'I'm your friend/not your friend' shit at the age of 15 to be utter bollocks that she wants no part of.
My DD too has no one to go to prom with and she has been SO looking forward to it but the girl she's been close friends with for months has in recent weeks moved over to the bitchy clique and DD wants nothing to do with all of that.
What's the answer? I'm not sure - but it's no fun for her or for me that's for sure and I totally feel for you OP. And for your poor DD.