Although demographics obviously are the driving force in measures of student achievement, it is possible for one state to do a better job than another relative to what it has to work with in terms of student potential. One interesting way to analyze the value added performance of a state's public schools is to compare 8th grade scores versus 4th grade scores on the National Assessment of Educational Progress. If a state improves from 4th to 8th grade relative to the rest of the country, this could be evidence that it is doing a good job of schooling (at least in the middle years).
The NAEP is also given to 12th graders, but those score are distorted by the large number of dropouts.
There are lots of data on the NAEP site, so if anybody analyzes it, let me know.