Actually, it depends on the roads and bus routes - at least around here. We've had school released early because of the threat of bad weather, but then again, we also have bus routes that run back up in the hills and valleys, and some of the low spots will be under four feet of water in nothing flat.
As far as tornado warnings, when I was in junior high school in Alabama, we had several warnings each year. Ended up with the desks piled against the interior wall with us hiding under them, the second floor and outbuilding students would be in the hallway, doing the duck and cover. Came in handy during the near-miss we had.