A few days ago, I would have been unable to comment. However, we were with friends on Sunday who's Daughter was in exactly the same situation at her scool. She was not challenged at all and it was recommended that perhaps a place at Grammar School would be the most suitable option for her.
They contacted BGS and she was invited to sit a test at the school and was offered a place the next day! She started this Monday. This was also for entry into Year 10.
I do not expect this is something that happens often but it clearly does happen. Perhaps contact the Grammar School direct and ask for their advise?
Hope the above helps a little