Yeah, if kids all have equal time outside of school (because poor kids have the same workloads at home as rich kids), equal access to resources (because poor kids have equal access to computers and internet access as rich kids), and we're relying solely on the kids motivation (rather than parents who can supply time to engage with their kids education, unlike the kids whose parents are working multiple jobs just to make ends meet).

If all that's true, then yeah, it's just the kids' choice of how they spend their time and effort, and NOT a proxy for wealth. But I don't think all of that is true.