As a parent of a DS who was bullied, I can feel your pain.
DS was excluded from his Year 7 friendship group very early on in High School, they followed him around, they called him stupid, they told him he was worthless. They tried very hard to ensure he didn't make other friends. He had his belongings broken and stolen, every day they emptied his bag on the way in and laughed at him, and so many other things I can't list because it makes me so mad (still).
He used to come home and cry and felt he was everything they said he was. He was so angry and scared and alone, they absolutely broke him, and he changed so much.
Whilst he wasn't ever back to fully being himself in High School, he did far better than I thought he would (as a person). He was genuinely happy BUT you can read some of my comments on other posts about his lack of effort (he was always worried about trying and failing and the bullies being right about him being stupid). Not trying and failing was so much better to him than trying and failing because the former meant he knew he could have done better ... he didn't quite do as well academically as he probably could have, but it's not the end of the world and he's now started college.
He did, however, have support from several teachers who went above and beyond for him, helped him and listened to him, put things in place even when the SLT were refusing to take action against the bullies. These teachers believed him and believed in him and did all they could to make sure he was alright, to boost his confidence, to ensure he felt safe.
I wanted him to change schools, he wanted to stay, he managed to make new friends and he has been happy, he has an amazing groups of friends now who helped him through the really hard times.
I thought DS was strong after coping with it all, but since his last day in May, he has found even more resilience. He is somehow brighter, funnier and more brilliant than ever. It was a journey he needed to make, he needed to see High School through and not run away, but now it's done, you can see it's like a weight lifted off his shoulders. I have second guessed whether I should have moved schools. But I didn't and he has made it, but I know he definitely couldn't have done it without that support network, and if your DD doesn't have one at school then she needs to be taken out of that situation. There is only so much you can do as a Mum when she has to walk back into the lions den alone.
I wish you and your DD all the best, and you do whatever you have to, to make sure she is OK and know that you will fight for her, no matter what xx