Do other small departments like Drama, Geography etc have subject leaders? If so, how much do they get paid? If you have a really good union rep at your school, who isn't a gossip, I would get a bit of advice as to what you can do. Things like the instrumental lessons are definitely not your responsibility - it is an admin task. Maybe you should put together a load of evidence, explaining the impact you have had on the department and present it to your SLT. If you can provide evidence, then it's a great deal more difficult to argue with. Year 8's just arriving...better go!