Yes, she doesn't have to make new friends if she doesn't want to. She might find that she just enjoys being in an environment with different people, though, without getting close to any of them. It's a good social skill to learn, to belong to groups etc. And if anything does happen with her small group of friends, it might be quite comforting to have a space apart. Teenage girl friendships can sometimes go through some difficult phases.
But it might be worth exploring what she is finding so difficult about it; if it's something really worrying her about doing it, etc, then finding ways to support her or make it easier would be good. Or if it's going to cause too much anxiety for some reason, then I also wouldn't force it, but have a more gentle introduction, suggest her friends come, make a buddy/mentor system, etc. Or if she is really embarrassed or feeling left out from her other group of friends, there might be reasons why not to make her. Perhaps she could take a little time to get used to the school move - but she might find it easier to do it now, rather than after everyone else has started.
But if it's just not wanting to, or it being too much effort, or too much change, etc, then I'd be insisting a bit harder, because she has some duty to do what she can to support the school - for a school to offer a selection of music lessons, it needs to have a culture of seeing music as important, and that can mean also having ensembles. If people won't join ensembles, it makes the musical culture of the school less, and thus less impetus for musical opportunities in general. And particularly if she is benefiting from the fact that she has the chance to have lessons through school - someone is organising that, getting the teacher in, arranging a room, arranging a timetable, etc, and possibly subsiding the lessons or at least having access to them at a decent price - it seems a small thing to ask that those involved give back by participating in ensembles. Good ensembles will help the school be known for music, will help good pupils want to come to that school, will attract good music teachers who enjoy conducting the ensembles or getting them involved in trips and competitions, etc.
Pupils who just take from the system won't be that popular with the staff or others, because there can be this implied sense of being better than everyone else. It might not be the case at all, but they could still get that impression. And that's not a great position to be in, when she might some day want to be involved in something or be chosen for an event or get permission to miss school to do something or have a reference.
Good school choirs and orchestras can also do tours and competitions that can be a really fun way to travel - totally different than travelling or living somewhere with family. And it could give her the chance to be involved with that.
Could some of her friends be encouraged to join too, even just for the first rehearsal or two? They might well accept people who don't do singing lessons too. Or the singing teacher could introduce her to some others, before she has to go in. If she wants to be a better singer, and do higher level exams, she might find that learning skills in choir will help with that - singing harmony will be good for her tuning and aural skills, her sight reading will improve, she'll learn some theory as a result, especially if she doesn't always sing the tune. Maybe just put it to her that way, that her singing teacher thinks she ought to do this as a way of supplementing what is happening in lessons. People at Saturday conservatoires are always made to do more than their primary lesson - they do second instruments, ensembles, choirs, theory, aural lessons, etc. - all because it helps them become more musical.