S/E - Use class time. There is nothing wrong with doing a 5 minute ensemble warm up and then heading to the cafeteria. Let the kids scatter across the room and you float. If you group solo titles work in groups of the same music to get more group instruction. You could easily do this a few times a week. This allows you to focus on individual sounds and technique which will pay dividends later when you are working on full ensemble concepts. The better the individual the better the ensemble.