That’s quite the gymnastic trick to roll yourself up in a mat and, with just your head, lean the mat against the wall upside down.
The mats were rolled already, standing up vertically, apparently behind the bleachers, IIRC. Video shows him walking in alone. Kids supposedly would keep stuff on top of the ends of the rolled mats, as they do not have lockers. He apparently climbed on top and fell in, and became trapped.
If you bothered reading....the mats were already rolled