DC Numbers Problem

For the first time since I've been doing this trip I've got a numbers problem. We've always been at a point where we needed 10 or so more students to fill up a bus, not at a point that we might have 10 too many. That might not make any sense....here's an example. Say each bus we take when we land in DC holds 50 people. Each empty seat causes us problems (costs money). We can't have 108 or 158 or 208, etc.... in other words we can't have one bus with 8 people on it or a bunch of buses that are 3/4 filled. We are at the point today (Thursday) that we only have about a dozen seats left before we start trying to fill another bus. Because of that we are going to put together a waiting list for everyone that signs up after we fill the current bus (I hate that idea). This is the first year that I've had to do this. If you're interested in going, please, please, please sign up asap. I don't want to leave anyone out.