Sorry, I meant in “in addition to your hypothesis” not-supposed-to-be-here could be considered particularly egregious to administration because the entire event is a good example of what can happen when a child is just left in their facilities without any system it place for him. If the kid isn’t taking part in the curriculum at that time, (or if the school isn’t fully aware that he’s even there) then he isn’t getting proper supervision. As part of a class, or after-school program, the kids are accounted for and supervised. Just loitering in the halls until a parent shows up, he’s not.
“Running in the halls and causing injuries” would be the kid’s fault for being reckless. By suspending the kid for “being on school property when he isn’t supposed to be” takes a better shot at the parents’ fault for leaving him at school in unsupervised conditions.
It’s just too bad you can’t suspend the parents – that would be more appropriate since it isn’t the kid’s fault that they didn’t make provisions for him.
Or give them a dunce cap and a good paddling.