If they can't play more than 16 or so measures without it falling apart completely, it's too hard.
You want something they can read big chunks of, but that will need polish/cleaning. Your 3-4 months away performance goal isn't realistic, either. You'll have TONS of lost days due to testing, pep rallies (yes, even in the Spring, and even at the JH level), snow/cold weather days, fair/show days, athletics, and the like. In a normal year, you should plan on only having 2 months to get ready for UIL. If you can't do it in twenty to thirty rehearsals, it's too hard. If you're *ready* and UIL is in three weeks, work on more sight reading stuff and throw in a piece for your Spring concert.
Most bands don't have 100% of their students in class between February and UIL, even in non-COVID situations. This year? All bets are off. Play what you can put together.