My son plays trumpet in the HS band. Last year, when he was in 8th grade, they(the bus drivers) pulled this "No instrument on the bus" deal with them. I chatted with the band director and he encouraged me to call the board office with it.
Coincidentally, this decree came after football season, when the players often cart their equipment to and from school. I mentioned this when I talked with our school lawyer. Her response was that the bus drivers said that the instruments were a hazard due to their hard cases. I responded with band was a graded class that required instruments and football/other sports were not graded and yet, they could bring everything and anything they needed. Response to that? Their duffel bags were soft.(!)
She said we, the parents, were responsible for getting the instruments to and from school. If we wanted, she said, we could leave the instruments in the band room, although she couldn't promise they'd be safe. She also suggested buying/renting a second to keep at home. I then suggested since band was actually a grade, that they(the district) should supply the transportation(IE a band bus) to the kids this rule affected. She literally LOL'd and asked where the money for that would come from. Wrong question to ask me!
Long story short, after a few weeks of being squeaky, my son got permission to bring his trumpet on the bus. I met with his bus driver and this principal and made sure they both knew. They did. I also let the other parents know who I talked to, what I said, and how the issue was resolved. In the end, this rule has all but disappeared. They modified it to say large instruments are not allowed on the bus.(basically, drum sets and key boards) Instruments that typically weren't brought on the bus anyways.