the good thing is the chances of schools getting hit with students still inside is somewhat tempered by most schools having hours of 7am-3pm or so, there really is only a short overlap between when students get out and when tornado ocrruences start to increase as afternoon heating can occur. of course , there is the threat of tornadoes hitting large amounts of buses as schools lets out, as i think happened in oak lawn Ill in the 60s.
I still think the biggest loss of life threat comes with a sporting event being hit by a large tornado. Just as an example, say you have 80,0000 people at a nascar event in kansas or texas, thats people spread out in the open over a 2-3 mile area, plus 1000s of people camping in the infield, plus that many crewmen, reporters, etc. Where do you put all these people in 20 mintues if a tornado is coming at that? this is where you could see deaths approaching 1000