Hello. I've only recently graduated high school so hopefully I can help with a bit of insight.
It's not surprising to realise that empathy sadly isn't innate in everyone, and is something that can radically grow with life experience. But more than that, mental health struggles are still heavily stigmatised, and misunderstood by people my age. Combine this with immaturity and high school, and you have a picture of someone 'only being attention seeking', or 'having a sook'. It's really rough.
From my experience in friendships, I'm lucky enough to have a pretty good understanding of how complex mental health struggles are. My best friend would feel dismissed in my friend group, because she would blame her rude behaviour to others on her mental health struggles which would really put people off. While she was struggling with her mental health at the time, in no way could this justify her behaviour, and she rarely took accountability for this.
In every young persons life, they will have more forgiving and understanding friends that are genuine, and others that are flat out cruel. But I think it comes down to perspective, from what it sounds like, your daughter doesn't have the kind of friends that would have the compassion to understand where she's coming from - this could be cruel, or it could be naivety and immaturity and they're perceiving that she's selfish.
If in any case your daughter has said something in the wrong way that have made her friends do this - being rejected the way she has is in no way her fault, and she deserves a whole lot better, friends who are understanding and compassionate. Remind her that not everyone is compatible, and that what her friends have done reflect infinitely on them, not her or her anxiety. I hope that they have the hearts to work it out with each other and are mature enough to take accountability. And if this isn't the case, then she is much better off finding new friendships and salvaging the ones that are worth it from the group.
Your daughter is only young, I've had counselling and treatment at 15 and it was very helpful for me. Please remind her that being excluded is just an awful part of high school, it doesn't reflect on her mental health struggles, and there is absolutely nothing 'wrong' with her.
You're an amazing parent for coming on here to discuss this, and it really seems like your daughter has an amazing support system and will move down a much better path.
I'm wishing you and her my best, Bella