In California, there is something called API number which is commonly used for comparing schools. But at higher end, the API is not a good indicator due to ceiling effect.
Number of national merit semi-finalists that a school produces seems to be a good way to compare a good school with another good school. Since students can only take PSAT once (in Junior year) and most people normally don't prep for PSAT, it may be a btter indicator than average SAT score (people can prep SAT and take multiple times). NMS semi-finalist name list for each state is released each year. Every one can see it to make his own judegement.
A similar approach is to check the number of presidential scholar candidates that a school produces each year. That information is available publicly. To be a presidential scholar candidate, you have to get (at least in CA) perfect SAT score in reading and math (single setting). Most school don't even have one. If you see unusal concentration in one particular school, then you know.
