@huy456 I can understand the dilemma.
Dd2 (Year 8) is definitely two to a keyboard (though not three) but I don't think it's actually doing her any harm for the one lesson a week that is the norm in most secondaries before they take GCSE options (she's not a pianist anyway, but plays three other instruments, so it's actually quite good for her to sit with someone else!).
If they take music for GCSE in a state school the classes are often very small as it's largely self selecting. DD1 is in a class of 15 for GCSE - plenty of keyboards (and all the other tech) to go round. It's worth asking about class sizes, and the levels of the children who actually take the exam - as that will affect the overall experience.
I would say that the musical performance standard is lower in my DDs' london comp than in the local independent schools (though this is London which skews everything). On the flip side, though, DD2 has a lot of love and attention poured into her because she is a bit unusual - that can be very good for a child's confidence. She gets the push and competition from her Saturday school (and I assume will from the london and national stuff this year), so I'm not worried too much that she's missing it!
There are plenty of extra curriculars at some state schools. My children could do a different ensemble every day if they wanted, despite it not being a school with music places. The issue is often that they don't want to, because it clashes with other things and there are only so many hours in the day.