They aren't her friends. If they were, she'd be invited.
I think you need to start there. You need to tell your DD that they aren't her friends. Maybe they hang out in school, maybe she sits with one or two of them in school but ultimately they just don't view her as a friend and they don't like her or at least 1 or 2 don't like her.
She needs to walk away from them. And that's so so hard for a young girl her age - but honestly, if she did decide to keep it breezy with them, smile, say hi but then take herself off to the library at lunch time, or take on a lunchtime club - just anything where she can say, 'I'm doing x, catch you guys later' and just repeatedly do this - then she'll likely make new friends, and these girls will fall by the way side (and will likely be quite intrigued by her new attitude and treat her better).
Have her distance herself from them on her phone - turn off her blue ticks on whatsapp. And teach her phrases when she gets a message to reply, 'sorry for delay- been out today' and other breezy replies that doesn't initiate a conversation. She just needs to pull away.
The point of doing it- is not so she can 'get in with the crowd' but is for her to distance herself from them.
They aren't her friends OP. She needs to recognise that, and from there start widening her social circle.
Not easy but necessary.
For the next couple of weeks get some cool weekend plans in the dairy - cinema, shopping, or whatever else she's into it.
But she needs to understand and accept that they aren't her friends and now is the time to pull back. Horrible for her because at 15yrs friendship groups in school are usually well formed and it can be hard to break into a new one.
School on Monday she needs to go in with a big smile attitude of 'I don't care, I'm just going to be nice to everyone' and 'I'm going to start up a conversation with someone I don't usually talk to'.
She has to brave it out. Even if that means eating lunch on her own. Again, I get it horrible for her - but if she can meet this with confidence, a smile, and an attitude of 'I'm fine all by myself' and she can show up with a friendly outgoing attitude, her world will change and it will change fast. She just needs to ride out this part.