9i know this will be irritating, but are you absolutely sure? the school near us that do early GCSEs only do 10/12. my dd is in a very academic grammar school and they do 10-of which only 8 have to be academic subjects. They have to beg if they want to do any more.
I would go in and talk to her head of year, explain about her dance commitments and seeif you can work something out. I would also be very careful about deciding she's not going to do A levels, though. Who knows what xoould happen over the next 2 years?