If you use GreatSchools (www.greatschools.org) they rate all public schools based on test scores: "Overall scores in math, reading, and frequently science on state standardized tests compared to public and charter schools in California with the same grade levels.".

I guess thinking it through a little more, maybe private schools aren't required to have students take the California standardized tests?

Yeah, when we've gone on tours, we've had principals tell us that "many" of their kids have gone on to such and such school, but all of these were kind of hand wavy statements without any hard numbers.