From what I know, no one did a bed check that night. This should have been the chaperones' duty, I think.
Most of the kids went back to the hotel, but not to bed. They stayed outside by the pool, or went visiting from room to room. It would be easy to fail to miss one of 140, unless the chaperones had assigned a special place to meet and check in with them the night before departure.
IMHO, The chaperones messed up bigtime. A responsible chaperone would have seen to it that the kids had all checked in with him/her, that their bags were packed, that they had their passports in order, etc. With 7 chaperones, each would have to check only 20 students.
I think the chaperones were out all night partying themselves, acting like a bunch of teenagers. Some Boomers don't want to grow up.
That is the problem. They weren't real 'chaperones'. *rolling eyes*