Does your admin expect you to walk students to the restroom during class? Or can they take a restroom pass and walk themselves?

Marching band rehearsal is an extension of class, same rules should apply.

We have a restroom key on a lanyard that is the pass. My kids know to take the key, take care of their business and come back quickly. Generally they go as a group during water break.

As someone else said, we cannot monitor 100% of behavior 100% of the time. If students are fooling around, apply appropriate discipline, move on with your life.