My niece and a friend's child also go to Stage School on weekends/after school (can't remember which one does what day) but that may also be an option. The instruments and band is a bit harder (although if they like brass there are a lot of community bands there).
Also wanted to say that sometimes literally one teacher with a bit of a passion coming in can turn this around - it might be worth talking to the school about whether music etc. is a direction they're looking at going in.
Good luck - I still find the "having secondary school options" thing kind of weird (grew up rural, was a non- Catholic girl... so one school available! Was fortunate that music, theatre and fairly wide range of academic subjects were covered.)