Have your rehearsal planned to the minute, keep their horns on their lips and don't talk so much.
Give them short, concise instructions when you cut them off. Don't do the "Ok, let's see here, Trumpets go to measure 7" bit".
Give general feedback and get them playing again.
Do this until they know what your expectations are.
Slowly, increase the amount of time you talk up to the point that they start talking and then it's " Ok, everyone measure 16, 1,2,ready, play"
Remember, these are not your kids yet. You need to make them yours and it will take some time.
Best of luck.