According to this study (http://economix.blogs.nytimes.com/2009/08/27/sat-scores-and-...), there's a clear correlation between students from high income backgrounds having higher scores versus students from lower income backgrounds having lower scores as well.
The idea that test scores are "meritocratic" is itself a myth. Wealthy people score higher on tests for very simple socioeconomic reasons that are not related to any innate merit but rather to the advantages conferred by an aristocratic upbringing.
Attendance at elite schools and the myth of the "meritocratic test score" is just the most recent form of upper class hegemony. It's a fiction created to justify hereditary rule. The far majority of wealth, power, and status is obtained through heredity and nepotism. The "meritocratic test score" myth is there to hide this simple fact.
