I was bullied. It's horrible. Ostracised and all the rest of it. They said I was 'ugly'. I wasn't, but it made them feel better.
Thing is, your daughter clearly can stand up for herself and even has the guts to indulge in a bit of bullying herself. She's no wallflower weakling like I was, physically beaten by other girls and boys for being 'ugly' and 'nerdy'. She's had an argument and come off worse. We win some, we lose some.
Hold head high. Ignore. She doesn't need to care about their messages - block their numbers, unfollow on social media. She can log any incidents of catcalls in the corridors or physical intimidation/violence, but so long as she keeps out of it and doesn't be 'a little shit' (that's kind of you) she will come out of it just fine while they, if they go too far, will be punished. If she doesn't give them a reaction, they will grow bored. Especially at 16 when they've got other things to think about.
But she can't play victim or take any moral high ground if she gets involved too, provokes, replies to their comments and so on. Once it's all printed out it will look like a group of kids having a spat and not like lots ganging up on one.
She's almost done with school. She just has to ride this one out.