The best have already risen to the top. We already have a top 10 who seem to do relatively well. So why, on earth, are we not simply copying them ???
The Minnesotas, the New Jerseys, the Massachesettses, even the Wisconsins... why don't we just take one of these models and install it everywhere? Are kids in California, Washington and Nebraska really all that different from the kids in the old Union states ???
I'm thinking not having anything better to do in the winter might have an effect on scores.
I teach in South-West Western Australia so our winters are cold and wet and the summers are quite dry and hot and it's quite interesting to see the effect the weather and climate has on students (it certainly has an effect on emotional and behavioural characteristics). It'd be interesting to see how comparative students do in different climates.
Some quick looking about only found an article on worker productivity, which isn't really the same thing: http://hbswk.hbs.edu/item/6975.html