I'm a teacher, & have done any number of residentials over the years.
Yes, we'd count them off the coach, plus they're in groups with a teacher responsible for usually 10-15 kids. Nearer to 10, I'd've thought, with this age group.
They might then be spread over different floors of a hotel to sleep, & the groups supervised by each teacher might be different over night because of male/female rooms.
Eg: I am responsible for a 'day' group of 12 kids, probably selected based on friendship or behaviour or activity options.
I supervise them all day, & get those 12 safely on the coach after the last activity of the day, off it back at the hotel, & send them to their rooms.
I am then responsible for a different 'night' group of 16 girls (one member of staff gets a night off each evening). They are in four bedrooms - hopefully on the same floor & close to my room. I make sure they're all in their rooms & patrol until all is quiet.
You'd have to have more than one staff member being really slack to lose a kid overnight - even if it's a dare & their mates are in on it. Practically impossible if not.
So the 'day' teacher counting heads off the bus is told 'oh Joe's already run in, miss, he was desperate for a wee!' & doesn't follow up by checking the room, AND the 'night' teacher checking Joe's room is told 'he's in the shower, sir.'
Meanwhile, the coach driver hasn't checked the vehicle either...
It does sound like no one was checking the checks were done. But I'm still suspicious that it could happen without the other students noticing an empty bed.
Normally they love a midnight 'OMG sir miss sir Ellie's been kidnapped miss sir!' drama...(Ellie turns out to be in her mate's room because she desperately needed a loan of her straightners at 1am).