This is a pretty cool opportunity. I'm thinking you could probably devise a system that works best for you and your students.

For instance, at the middle school level -- do your students discern what is really good and what is not? Or are they great at just following the "plan"? So in other words, if you have a future educator in your class, I'd recommend allowing them to run class and even hear sections of the music in order to improve it.

With that being said, I have a feeling most middle school students would be better at managing the classroom... i.e. "Let's all do our warm up, now" and "Take out the march, we're starting at measure 32..." then turn on the met and so forth, without really giving feedback.

You might maybe prefer to let your students use that time in little mini sectionals, and if so, give them specific places in your music to work on at specific tempos. The advantage of the sectional time rather than full group time would be... there might be a rock star kid on his/her instrument who would know exactly what to say to his/her peers on the same instrument, but not know what to say to others on their respective instruments in different family groups.

I love the idea of giving kids some opportunities to practice being a leader. Good luck as you develop your personal plan!