CFK your niece sounds a lot like me at that age, and her parents like mine. No-one advocated and the result wasn't pretty. Actually it was really ugly for a long time.
She's lucky to have you on her side - are there any other adult or older teenage mentors you can call in too? It's so important to be around like-minded people to develop a healthy sense of personal context, and much easier to open channels of communication along which wise advice can be imparted. The parent-child relationship can be difficult, especially when judgements are already being made. I still remember it well.
If you don't have any luck with her preferred outcomes, can you coach her on different approaches she might be able to take with teachers? (assuming you can't do this yourself as 'only' an aunt) Eg propose different assessment mechanisms that she might find more interesting. Does the school have any ties to nearby universities that she could benefit from?
I really hope you have some luck. School at the moment probably isn't relevant to what she's interested in and what she can do, but it's so very relevant to setting yourself up for a path through life that will be interesting and rewarding.
