Students who practice reading, and practice writing more through repetition will be more successful, and learning how to play an instrument is pretty much the same. Do students only practice writing every other day? Do they only have reading lessons twice a week? Obviously not because that would drastically hinder their ability and progress.
Moving band to twice a week will hinder the students ability to learn that instrument the same way it would hinder their reading ability if that were cut down.
I would also emphasize that 5th grade band isn't really a "band" more so, a "how to play an instrument class". Talk about how beginner band is pretty much the most important year for developing a students playing ability, and cutting down the time at that level is incredibly damaging. Some principals don't realize how different the first year of band can be compared to every other year in band.
My 2 cents is, if the principal doesn't budge and moves your beginners to twice a week, is to stop doing 5th grade band and see if you can improve your beginner situation by starting in 6th grade. In my own opinion, daily beginning band one year later (especially if you can divide the instruments in any way), would be significantly better than twice a week 5th grade band.
Also, do some research, find some other 2a bands in the area (or more effectively, your football district) and ask about their beginner situation. I bet their is a direct correlation to the bands UIL scores and their beginning band situation. Data is important, and the better you can show that not daily beginning band =/= success, the better.