Here is another excerpt from the book that this article is taken from, this one talking more about the disaster at Okawa Elementary School. Be warned, it might be difficult to read.
https://www.theguardian.com/world/2017/aug/24/the-school-ben...
https://www.theguardian.com/world/2017/aug/24/the-school-ben...
It was bureaucracy that stopped them from going to hill... but thats simply because the bureaucracy was doing what it does to stop panic and chaos. In a situation where the event was prepared for, having a small group of kids yelling orders and running off is exactly what you don't want. You don't want conflicting orders running around; else (if the bureaucracy judged correctly, and went to the hill), you'll end up with a group of kids and teachers or parents not following orders and staying in the school instead.