This is where student leadership really comes into play. I can hand a rehearsal off to a drum major with little prep. Really comes in handy when I get pulled for a short-notice something and admin sends someone to "cover my class." Most of my section leaders can handle running a sectional, if something is going really poorly I can send a section into the ensemble room and have their section leader fix problems.

You'll have to adapt this to middle school, but some things we do:
Starting about now, those who are interested in leadership for the fall rotate through running warmups. I'm there to give feedback, but they run the met, call up what's next, etc.
Lots of listening in class. At the end of a rep, ask a student what they hear that we can fix. Sometimes they hear things that I missed. This also gives you a clue of who will be good as section leader.
Post-UIL, we do even more student-led rehearsals. I'm in the room in case things go off the rails, but the kids are running it. Especially kids who want to be drum major. At first I give them a detailed plan including measure numbers and tempos, later I give them more freedom to decide what they're going to work on.
During marching band, section leaders are running sectionals constantly. This gives the leaders some experience, but also the other kids get used to listening to their peers as leaders. An hour of student-led sectionals helps us make huge strides when we come back together.

Like anything else, if you throw something new at them while you're gone, it's likely to crash and burn. Prep it ahead of time.