A word of caution re music GCSE - it's not necessarily a great measure of how seriously music is taken at a school. Many musical children, as I am sure you know, will do Grade 5 music theory - you need it to get beyond Grade 5 practical with ABRSM; they will therefore not necessarily want to take the GCSE, which can have a heavy contemporary element and involve lots of composition on computers and the like.
Gloucestershire Music run 'fast track' theory courses either over a week or weekend to get pupils up to grade 5. A good way to get it over and done with, though not necessarily a method that will help the child remember the theory when they actually need it.
At Stroud High, the music department have keyboards linked to PC's running Sibelius notation software (2 pupils per keyboard in years 7 &
. They get really good results in composition, in a very enthusiastic music department. On the open day, my DD2 had great fun playing one of her G4 piano pieces and watching the notes appear in Sibelius, then replaying what she'd typed in. I hadn't the heart to tell her that we can do it at home on the electric piano & Sibelius on my laptop, but it's too much faff to set it up (I tend to enter notes using the computer keyboard).