(To clarify a little further, although school in the UK is currently only compulsory till 16, anyone wanting to go to university stays on till 18. The word "high school" can mean several things but usually means the school a child starts at 11. SATs are nothing to do with anything you use the same initials for in the US. Yes, the US one-science-per-year thing seems very odd to the rest of the world!)
OP, your daughter sounds gifted but more importantly troubled, to me. I'm in the UK but don't know much about what you'll be able to get done. Is your DD under
CAHMS? If not I think that's your first stop; I'd be a lot more worried about the failing non-preferred subjects and being unhappy in the first instance than with the giftedness - it'll be difficult either to demonstrate or act on the latter while she's in this state. I would also recommend talking to someone at
NAGC.
Also, is home education an option? Clearly something needs to be done, but she doesn't sound in a stable enough place for radical acceleration to be ideal, to me.