Chocorabbit that's terrible. Both of mine want to do music GCSE, and my year 9 has already picked it. They go to an ordinary comp, in an area where most really musical kids are creamed off by either the state academies with 'music scholarships' or the private schools with real scholarships, so you wouldn't maybe expect the best.
But there are plenty of 8s and 9s in the music results and I think everyone learns in the music lessons. In Year 7 and 8, everyone takes music and everyone is expected to learn to read staff notation, for a start.
There are keyboard tasks in lessons (there are usually four skill levels to choose from on any task and DD2 who has just finished year 7 and who is about to take Grade 7 in a non-keyboard instrument, usually does the second from top - so there is differentiation), and basic composing on garageband. And quite a lot of singing.
Once they pick music GCSE it steps up. DD1s classmates are mostly grade 4 and 5 in year 9 (because it is self-selecting who takes it) - which is fine for GCSE as you don't need to play anything harder than that - perfectly possible to get grade 9 in performance with a grade 5 piece. They do theory, which is at least as hard as grade 5 theory in Year 9, and some pretty tricky listening papers - writing down notes onto a score from listening to them for example. They learn to compose in different styles.
In extracurricular terms (when no covid) there is a swing band, orchestra, flute group, cello group, two choirs and a madrigal/chamber choir group and a specialist teacher who takes small groups to do more contemporary bands each day after school. Instrumental lessons free for those on FSM, cheap for everyone else.
It's honestly seen as a very ordinary local school - and somewhat looked down on by those who like a shiny academy and who cannot work out why DD2 picked it when she could have gone for a scholarship elsewhere - but the music seems to be the least of our worries. What a shame it isn't always like that.