I don't know about you but filling the bathtub is my instinctual reaction to anything that goes wrong. Job going badly? Fill the bathtub. Medical emergency? Fill the bathtub. Lost my house key? FILL THE BATHTUB.
Also a high school will have massive water heaters and a chiller..
I live in Japan as well & told my family the same a long time back. There aren't that many other specifics you can plan for, with the family typically spread across the city on the average day, but at least grab a bathtub of clean water to see you through a few days till things settle down.
That's right, BUTHTUB