This study by Andrew Coulson (or, perhaps I should say: Andrew, Son of Coul) at CATO is an impressive piece of work. A few years ago, he published a study showing how test scores in math and English have been completely stagnant (I believe using the NAEP) since the 70's, while at the same time the inflation-adjusted money we are spending on education has skyrocketted. In short, he showed how spending on education is not the same thing as actually educating, and did so in a spectacular fashion.
Now, he is out with a more-detailed study (using the SAT--but washed of any socio-economic or "who takes the test" baggage, then compared to the NAEP), which shows the same results on a state-by-state basis.
Here's his graph for Massachusetts. Most states are similar.