My DD has also gone through some doubts with friends, thankfully nothing as bad as the OP's DD.
She is at an all girl's grammar too and is Y11.
She developed a lovely set of friends throughout the earlier years, but last year, one of the girls joined a different group, and DD straddled the two, but felt closer to the girl who left the group, and spent more time with the new group.
She is not really close to the 'nice' girls in the group, but gets on with them, but she was closer to the girl from the first group, but she then became closer to another girl in the group and they started mocking DD. They went to get lunch, DD was last in the queue, then they wandered off whilst she was getting hers, leaving her alone.
She noticed that no one has posted in the group chat for weeks, but they often talk about things they've talked about. So it's clear they've set up a new one, but excluded DD from it.
They still try to 'use' DD, e.g, if they go to a clinic at lunch, they ask DD to go with them.
Luckily, she spoke to me a few months ago about the mocking, and how she felt more secure with the first group (luckily she hadn't cut ties with them), so I encouraged her to spend more time with them.
She found out herself, when another girl died. DD was really upset and had the next day off school - not one of the second group reached out to her, but the first group did. I told her then, that is how you know who your real friends are.
That said, I was teased by a girl at school. I reconnected with her in our late 30s. She was the most supportive person to me, when DD2 died, so people can change, but group friendships are especially difficult at this age. They can turn on a knife's edge, and generally there is one who is the most dominant/influential, and sometimes the others go along with it, so they don't get targeted.