I'm reminded of my military days. Whenever we traveled by bus, "Cattle Car", etc. upon arrival at the destination we'd get into formation and COUNT OFF. If 76 bodies went in, 76 better come out.
Why can't these school vans do the same thing? Doesn't have to be as extreme as a military style count off. Why not have all the kids stand by the van and the driver or teacher count them? If 15 kids were on the van, then 15 kids should be standing by the van.