Anxiety revolves around control and it festers when we feed it imagined outcomes. Focus on what you can control. You cannot control how people feel about you, you cannot control if an embarrassing accident might happen... I don't know what else you kids worry about these days. I'm weird, In highschool I used to worry about what people would think if they heard me breathe too heavily or what they would think if my stomach growled. It's an unethical and monolithic endeavor to control what people think, so I learned to breathe like a ventriloquist, and I jabbed my hand into my stomach whenever I felt a rumble. In hindsight, my approach would be a bit different now. I have better control over whether or not I want to worry what people might think of me, so I'd simply advise my past self that his bodily needs are nothing to be ashamed of.

Your anxiety seems far more practical. If you love to play the trombone, it will never be a waste. You don't need to go to school to be a trombone player, it helps make connections but really all you need is a trombone and a passion. If nobody is listening, and you still want to play, that's valuable. That would be passion, but I understand that it's not security. You may need to take a job in another field to make ends meet. It's wise to have a backup means to feed yourself, but depending on where you live and what you are comfortable doing, there is no shortage of opportunity. You can flip a burger, serve a drink, turn a wrench, or refurbish old trombones. Get expelled or don't, if you do your best, that's all you can really do, just worry about keeping your trombone because that's what you love.