Depends on what you mean.

Respect for the process of learning music? Respect for rehearsal etiquette? Respect for their peers in their commitment to the ensemble and their craft, (this is a whole different discussion because I've seen a lot of directors NOT respect the students and their time causing more problems than the kids)? Or respect to you as the educator in front of the classroom?

Short answer is being calm, quiet, and consistent. Don't lose your cool, blow up, or make exceptions that can be construed as favoritism. Be prepared with a plan and do what you say you are going to do. Sometimes less is more so avoid being wordy or argumentative. Be very matter of fact.