We don't need numbers here, logic is fine. If it were a proxy for socio-economic status, there would have to be some way for the score to adjust if the student

We don't need numbers here, logic is fine. If it were a proxy for socio-economic status, there would have to be some way for the score to adjust if the student's parents suddenly went broke. Since there isn't it cannot be a proxy. Perhaps you meant it correlates? So what? What if it correlates better with success in college than pure socioeconomic status, which I'm fairly sure it does? Should we just use the less accurate one instead?