You cannot exclude wealth, both family and neighborhood, as a proxy. I did not come from a rich family but we were in the district for the public school which

You cannot exclude wealth, both family and neighborhood, as a proxy. I did not come from a rich family but we were in the district for the public school which included a lot of professional class parents. The school offered a lot of AP courses. If you do well on the AP test then some colleges would accept that as college credit. This would make it easier to get a double major. However, not everyone could afford the AP tests, and schools from poorer neighborhoods, or from smaller districts, didn't offer as many AP courses.