High school is fraught with danger. I really never understand why it is touted "as the best years of your life". Crimminey - it is the worst.
I have a son your daughter's age - just finished 10th - and he has discovered that friendships move and shift throughout elementary, middle, and high for no apparent reason. I keep an open line of communication with him, encourage him to invite those kids who he is friends over, and encourage him to continue to be polite to those who have moved on to other friendships. I do not intervene in his relationships expect to talk to him.
At this age, to have mom talk to the other kids, or parents, coaches, etc. may well cause more harm than good. It seems that at a far younger age this the trend is to let kids work it out on their own - despite the fact that do not have the skills to do so. So, you need to ensure that your daughter has the skills to work it out. And she sounds like a strong young girl who knows herself - so half the battle won there.
Do help her build other strong friendship by setting up summer activities with other girls, have a summer time BBQ and invite everyone she hangs out with and then some, etc.
Ditch the Twitter account - I mean really, in this case avoidance of the nasty grams may be the way to go. If the mean girls know they can not get to her that way, most likely they will stop using that medium. If she has a FB account - monitor it to ensure they are not cyber bullying her - if they do, then you have a case to take to a higher authority.
But basically, keep doing what you are doing. Be there for her and continue to mold her into the strong woman that she will become.
Hugs