Something's clearly wrong but, having had the responsibility of looking after kids myself (during my teaching years) I know just how easy it is to make one silly mistake.
For example, I organised a 10 day football tour to Germany. We counted the kids on to the coach at the school gates and counted them off at the motorway services. Then we counted them on again before counting them off at the ferry terminal and then counting them onto the ferry. For 10 days we counted those kids on and off the coach in France, Belgium, Germany and Luxembourg, at sports grounds, hotels, fairgrounds and just about everywhere else. Then we counted them on and off the ferry and back onto the coach before setting out back to Sheffield. Our final stop was at Leicester Forest East services where, for the one and only time in 10 days, we forgot to count the kids back onto the coach. We were halfway down the slip-road onto the M1 before anyone noticed that there was a kid missing! It's easily done!