The children were not at school.
When the bomb went off, Mrs. Stevenson's grade four class was on a field trip to the museum. Luckily for them, the museum had a bomb shelter underneath, paid for by a very wealthy and very paranoid patron.
The parents all rushed to the school, frightened out of their minds. All the other kids were delivered safely to their families, but all the parents with a fourth grade student waited anxiously for their children who never showed up.
The principal tried to comfort the wailing mothers, while the fathers were standing around angrily, blaming anyone they could. Only Johnny Littlemouse wasn't in the shelter. He'd forgotten his permission slip, and was safely delivered to his family.
The kids returned later, after it was determined the bomb struck nowhere near the town.