One thing I've wondered about regarding the 1000' measurement is if we should take elevation changes into consideration. I would argue that we should, because the wording of the law says, "within a distance of 1,000 feet from the grounds."
Say we couldn't carry firearms with locked containers through a school zone, and say you were transporting a firearm on a plane, and the plane flew through a school zone (but at 35,000 feet in the air), would you be the school zone? I think we can all agree that you would not.
But taking that logic a step further, one could say that while I may be 950 feet from a school zone, that since the elevation change is 350 feet from my position to the school that I am not in the school zone since when you calculate the hypotenuse of the distance you can see that:
950^2 + 350^2 = X^2
X^2 = 1025000
X = 1012.4
As we can see, I'm a good 12.4 feet outside of the school zone.
While I wouldn't bother using this logic when determining if I could go to a certain place (I'd rather err on the side of the typical horizontal only calculation), I would bring this up as a defense if I were to be prosecuted and there was significant elevation change present.