I have worked with over 20 different bands and drum corps. Rehearsals work best when you have a plan and stick to it. Don't get sidetracked and make sure you know how long you will rehearse something. Spending an hour on 8 counts really reaches the limit of no return very quickly.
One person should run the rehearsal, this can be the person on the ground or in the stands, (tower, pressbox) but it does work best with the upstairs person running it. That person sets the pace and has the power to do things over or not. They should have the field staff give input to students and make their own comments, but keep the comments short and relative to what is being worked on. A rehearsal like this is NOT sectional time. Sometimes a wind tech might want to turn it into one. The leader should take input from the staff, and if you have a good professional working relationship compromise during rehearsals can be reached. The person is a rehearsal leader not a dictator.
As much as we respect our student leaders and value their help, there should be NO talking on the field from the performers. One school I work with has the hardest time with this. The students are very, very smart and often want to know "why" things are being done. Often you can satisfy them by clearly stating this in the corrections or instructions. Often student leaders will not have the same priority. They see a diagonal that is not working from their view, but from upstairs it always forms fine and the more they yell, the less productive it becomes. So, there should be NO talking from students on the field. You can ask if there are questions, and group responses are good, but 20 chiefs and no indians is a bad idea.
If you start rehearsal on time, END it on time. You expect students to be in place at 3:30 and they are, they should expect to be let out when they are supposed to be. If students are wasteing time, you should still end on time. If you dont accomplish what was set out, tell them. Communicate cleary and ahead of time. I.e. Do not let them dictate rehearsal time. "you guys were 10 min late, so we will finish 10 min late" What if they come 30 min late the next time? Stick to a schedule, you are the professional.
Lastly, if you are going to "talk the talk" -Dedicatiion, commitment - hard work- energy - excitement - being on time- going the extra step- remembering your EQ, you better "Walk the Walk". Everyone is human and a band director might forget his set book once, or misplace the extra batteries for the head set, but if you do this over and over it is a sign to the students that you might not be commiting as much to the program as they are.
Focus and have fun.