That is not a very good idea. If money given by government is tied to test scores, schools have more incentive to let students cheat than otherwise, for the high scores. You'd be trying to sell them something that at the very least, lowers overall test score averages.
So you sell this on the federal level, to let the DoE make sure the state level schools are honest. What's even better, you can charge a lot more because the DoE has a bigger budget.
