1. Teach them to come into the without talking. If they don't do it, start over in the hallway. Take as many times as take to get them to do without talking. Give them a timer to get in the room, get their instrument and sit down. If anyone talks go back to the hallway and start over. Could take the whole period, that's okay, you have to establish procedure/habits.
2. During rehearsal, if they begin to talk tell them "in band we practice what we can't do. The first thing we have to learn to do is be silent so we can listen to instruction. We will practice being silent for ____seconds." If someone talks start the time over.
This all takes time but thats okay because in the end it will save time. Also, be very matter of fact about.