I'm not sure exactly what a parent could do to prevent this but I thought I would throw this out there as food for thought. Personally, I pick my kids up but I know that isn't an option for everyone.

We had thunder storms on Monday afternoon with lightening, 45-60 mph winds and small hail being common. Some areas had higher winds and there were some areas that had golf ball size hail. This is the same storm I had mentioned in another thread that had some local flash flooding as well. IIRC the area where I live got 6-8" of rain in an hour or two. I was supprised that they were putting kids onto or letting kids off the buses during the storm. A five year old got off at the wrong bus stop and was missing for about an hour during the storm. I was picking up my kids during the beginning of the storm and we experienced 50+ mph winds and pea to dime sized hail.

http://www.newson6.com/global/story.asp?s=11180266