I have my students rate their rehearsals from 1-10 at the end of each rehearsal. We then talk about the best parts and weakest parts. In the following rehearsals I will find a "great" moment and call it to their attention. "Hey gang, the rehearsal 'feels' great. Let's keep it going." I use the awareness of how the room feels; calm, quiet, mentally centered, etc. Then if we are challenged, I mention that the "classroom doesn't feel correct". Generally kids will work to return to the correct atmosphere for success. I often refer to the band hall and even the marching field as a classroom. Educationally, administrators, coaches and parents understand that we are teaching and not just playing around.
There is also a strong necessity for students to know the order of events of the class time. Always close class with the expectations of individual practice and the next rehearsal.
Remember to "lead toward success" rather than "yell and scream" when things are going poorly. Honesty goes a long way. If they know you are working/teaching rather than controlling them, things are a lot easier.
Individual praise and group praise works well. I have special stationary that I send home to parents praising efforts for our band's success. Newspaper articles and school announcements help to inspire kids to want to be in class. Going back to my first comments, this makes them want to raise their rehearsal ratings because it is their responsibility rather than my control for improvement.