Here's another site you might find helpful http://www.ggusd.k12.ca.us/SARC/sarc.asp
Here is what the Trulia site says:
GreatSchools ratings are scores on a scale of one to ten (lowest to highest) that indicate how schools performed on standardized tests compared to other schools in the same state. You should consider other factors like teacher quality, school culture, special programs, etc. Ratings may not be available for all schools as the test results may be unavailable or incomplete for certain schools. Private schools are not required to publicly report test results so these schools do not have ratings. For more details about the ratings and methodology, visit Greatschools.org.
So it's as compared to other schools in the California.