I think if anything, college will be the best place to find a great number of Kindles being used. Then, it would move to private high schools, but that might be awhile because there are a lot of things you can't do on a Kindle, that you can do on a laptop, which is what most private high schools require.
I would think hs would be better just because you can't write on the text books (at least for public schools), but in college you can. When I was in college, I always wrote on my books.
I do agree, though, that it's still going to take a while for this to catch on (with some much needed improvements) because it's just not that quick to use as a pen and paper or a laptop.