You could do it earlier, but performance and knowing music theory aren't the only 2 things you need. You also need to:
- Compose 2 pieces (at least in my exam board).
- Learn case studies from several pieces.
While this generally isn't too difficult, it takes time. It would make a lot more sense just to take Music for a GCSE in Year 11, as colleges don't really care when you took your GCSEs, but more what you got in them.