Frankly I think it's ridiculous. You pay tuition, so is it high school or higher? Whatever, french horns are as hard to play in tune as any mellophone (which was built to be a substitute for a french horn which doesn't lend itself to marching), especially by someone who has just begun to play. It is a totally unreasonable request from the director, who wants that part to be heard, but is unwilling to compromise re. how it is done, at the expense of your chops, your face (the kind one saves) when you sound awful in public, and your stress level. I'd tell him it's mellophone, flugelhorn, or you don't do it. You did your work and raised money and are PAYING the difference. Now if he was paying YOU that would be different. It would be better to have no fhorns than someone kaacking or playing out of tune. Your best chance, tell him, of providing what he wants, is to play the mellophone, which needs no transposing at the last moment, and which you'll be able to lip in tune with your trumpet background. One of the most difficult things to learn in life is to say no. When you say you HAVE TO play it, what exactly transpired in your dialog with the director? Did he order you to do it?(No) Did he ask you? Did he raise the concern and you volunteered? If he asked, then you could have (and probably should have) said no, but didn't. If you volunteered you demonstrated that you are a team player. Either way there should still be room for compromise. So the adult thing to do is tell him exactly what you are willing to do to help out, and if he won't accept it, that is his choice, and you are off the hook. He can ask a t'bonist to sub instead.