This is a patently absurd argument. If you think something might be a bomb you don't automatically believe the bomber when he says it's a clock.
If you think something might be a bomb, you evacuate the school, you call the bomb squad. No one involved thought this was a bomb and this is evidenced by the fact that no actions were taken to disposed of the bomb and get people to safety.
It's fascinating that you know exactly what everyone thought.
In any case I bet they thought it was a bomb at least as much as those other school officials thought the pop tart gun was a gun.