WARM-UP: MUST START ON TIME!! This is the time when you teach your students how to play. We have to refresh rehearsal skills, posture skills, foot tap skills, reading skills, listening skills, and counting skills. All of that is getting the student ready to revert to habits that have been developed over time.
And we must warm the instruments, the fingers, the tongue, the brain, and the heart (the fun part). If the warm-up is done well, there are parts of every piece that do not require rehearsal.
TALKING: As John said, you should have someone time you. Take credit for playing time when even one student is performing (counting, playing, singing). But find out how much time is spent with none of that happening. It is shameful how little our kids get to play in band. Rehearsing a piece of music as described below helps me to talk less.
1. Band demonstrates what it can do
2. Stop for the director's briefest possible correction
3. "Show me" (students demonstrate understanding of what the director just said)
4. Adjustment or praise (director response to the demonstration)
5. "Show me again"
6. "Thank you! That is just right. Now mark that in your music, please."
Of course, it sometimes takes more "show-mes" than that, but the simplicity of the monitor and adjust strategy still applies.