It's an interesting cultural difference, but over here the school building would have a permanent ladder so that you could get down from the roof. The ladder is contructed typically so that it's not available from the ground level, so it's difficult to get up, but getting down isn't much of a problem, even if you need to drop a little bit at the end. It's not enough of a drop to cause any harm. You could call them emergency ladders, so that if the building is on fire, you won't be stuck on the roof even if the firefighters didn't appear immediately with their boom lifts. I guess nothing like that exists in Japan.