The way Greatschools rates schools is by using data on average test scores of students in that school, compared to others in the same state. 1 is lowest, 10 is highest, so 6 is slightly above average. (1 represents the lowest 10% of scores, 5 the mid range roughly 50th percentile, and 10 is the kids scoring in the top 10%, or roughly the 90 to 100th percentile.)

It's also important to realize that NCLB testing requires most students to take the tests, even those subgroups who might formerly have been excused, for example, students for whom English is not their first language. It's possible your district has a bunch of highly performing kids, and one or two groups whose needs aren't being met as well, resulting in the overall average score of 6...you should ask your principal about the school's test results, if you can't find an explanation about them online through your state's Board of Education website.