Greatschool "Research and Compare" feature is useful for comparing the performance of public schools within a given state, but it is widely known that state NCLB tests vary considerably in terms of their difficulty.  (One state may claim 80% of their second graders pass the math test, but they're only expected to know simple addition.  Another state may say 75% of their second graders pass the math test, which sounds worse, however that state may expect the children to already be doing muliplication.  It's hard to compare one state to another, because it's like comparing apples to oranges.)

National Assessment of Educational Progress (NAEP) is the only group that tests across the U.S., and provides a standard basis for comparison, also known as "The Nation's Report Card."  Their information points out the disparity between how states claim their children are doing in math and reading, versus how students are performing across the board...Here's a site that shows State NCLB testing comparisons to NAEP: www.time.com/nochild  Here's a recent Greatschools article about NAEP: www.greatschools.net/cgi-bin/showarticle/1200

Here is yet more detailed information that points out educational disparities from state-to-state within the U.S.A.:
It's an excellent report, and well worth your time to look at:  From Cradle to Career: Quality Counts state reports 2007.  Each year, Education Week releases its "Quality Counts" report.  It includes a look at how well states are doing in helping students make successful transitions from preschool to elementary school, and from high school to college. This web-based report includes several compelling charts and graphs, as well as commentary from experts. pbsmail.org/ct/w12w-rF1OXl9/fromcradle 

If you enter your state's name, you can get amazing details of how it compares, educationally, to other states in the U.S.
See the sections that say "State Highlights" and "Chance for Success" and "Supplemental Ranking Table."