@yodaforpresident DD2 started it off timetable at her old school official and afterschool - however, of the 8 that started there are only two left doing it apparently and no idea how they will do.
At her new school, most specialists take it in Year 9 unless they join later - and that includes those who join IN year 9, so they do it in a year.
However, DD2 joined in Year 10 so is doing it now. They pick a different board for those who take it older, reckoning it is a better fit for those pushing straight to A-Level
For your DD, performance she'll be great and 5 theory is a great starter - but depending on the board the analysis can be quite heavy - so some of the specialists in DD2's year got a 7 in those papers in the mocks despite being some of the best musicians in the school.
You have to know about your set texts! DD has had to work on this - Killer Queen did not come as naturally as Music for A While.
Composition, as @comefromaway says, has to be in school for assessment reasons, so it depends whether the school is willing to facilitate.
It's certainly harder to do alone than it used to be, and performance is only a third of the mark (and half of that is a duet/ensemble so you are reliant on organising it) so you can be a brilliant player but not do that well.
I suspect at an indie school with good music they'll be able to handle it. But do also think about DD's sanity as GCSEs approach - do they really need all of them? DD2 enjoys being able to things other than study and the pressure does mount up!
Your DD2 has got a while though I think, so you've got time to consider all options?