I was once told by a wiser, older band director that if it cannot be done in class it is not getting done. We do not have to play the hardest thing available. Do what your kids can do and do it well.

Play something easier. If the students want something more challenging they will step up. If the students don't then they will not. Just play whatever they decide to play to the best of their ability.

How long have you been at your school? Are you expecting to much from them? Are the students involved in a lot of other activities?