I'd show up, bring my instrument, lock it in my locker, and show up with it a couple of minutes late to band class. Peer pressure and bullies are cruel, I know, and I'm sorry that you're having to endure that. I was bullied too... I always root for the underdog to this day, though.<br />
Think about all possible outcomes of your actions. If you rat, then your teacher will probably walk into the classroom and say, "I heard there's a mutiny". Watch how quickly every head turns to look at YOU. It's not your job to protect your teacher, after all; and he will handle it his way. Just have your instrument with you that day. That way, it can't be said that you were a participant.
They threatened to destroy your flute????? That's awful!!!!
What the *******... I knew Band had a lot of drama, but wow!<br />
Short answer: Do what's important to you. Bring your flute. If this mutiny is really a big deal, your instructor will appreciate it enough to take care of you. If it isn't, the few people who left their instruments will look like idiots for not bringing them, and won't bother you at all.<br />
Long answer: Stop downing yourself for something as impermanent as grades. What matters is what you love to do, and if this is important to you, than do the smart thing and don't let people threaten you into selling yourself short.
Sometimes the best thing is to just sit there and be quiet. This is true if you have no real power in a situation, and you are caught between two forces, one legitimate, the other childish. <br />
The other good option is to not be there at all. That raises its own questions. You can't just stay home, can you? You have to have a better, preferably documented, excuse. Jail comes readily to mind. Depending on your jurisdiction, a mental ward might be a better bet. I know a thousand ways to get arrested, but some of the less outrageous ways include urinating in public and being a public nuisance. <br />
By doing that, you have an easily documented reason for your absence from the rebellion, but, as a minor on such a low misdemeanor, you get a slap on the wrist, ie, counseling. You can tell your counselor that your band mates are sheep abusing sexual predators, and you are scared. That should take the heat off you and go back to band practice. <br />
Disclaimer: These are just ideas to lighten up your situation. Please do not get yourself arrested. I really do not need an ambitious prosecutor, especially a British one, coming after me. "TheSquirrel clearly stated --- " No, no one needs that scenario played out in the courts.
I'd leave an anonymous note for teach then join the mutiny, or, stand up for what you beleive is right no matter the consquences. Not easy. Do whatever you can live with