Right, it's only a correlation, not a guarantee. And people who don't end up part of the correlation have maybe their best shot of becoming successful (assuming

Right, it's only a correlation, not a guarantee. And people who don't end up part of the correlation have maybe their best shot of becoming successful (assuming they pursue a career requiring a degree) via test scores. Anything else likely has a much stronger correlation with SES or might be very noisy: extracurriculars require money and parental involvement, high school grades can be worthless depending on the school, filtering by school rules out a lot of poor people, etc.