I would hypothesize that SES probably correlates very heavily with grades at the very bottom where you have things like abuse, abject poverty, children supporting their own parents, lack of access to quality nutrition, violence inside and outside the home, etc and even a genius would be swimming against the current. But middle class and above, I would expect the curve to flatten out. In my experience super wealthy people are more lucky than clever- it doesn’t take much brains to make more money when you’re already rich.
So, the goal here should be to identify smart kids in terrible situations, while not allowing rich kids who aren’t actually that talented to game the system. That’s the goal. And of course there will also be genuinely talented middle class kids and you don’t want to ignore them either.