here in louisiana we get that a lot bc of the hurricanes and how we have extreme heat. if we have tornado warning or theres a tropical storm or any hurricane, they cancel. if it snows, they cancel, if the temperature reaches the 100s they cancel... those are just a few reasons here...
Usually due to weather forecasts that warn a storm in the first place, teachers don't want to take any chances. Plus if buses are off and not running, it's a greater chance of the school not opening, since it's not worth to teach a small amount.
so they just call it a bad weather day for all kinds of reasons
It's just to be on the safe side.