For example, it has become fashionable in liberal circles to argue for abolishing the SAT because, while it is not intrinsically racist, its results reflect our racist society: people with less access to education and tutoring perform poorly, and this results in measurable racial disparities. Problem is, the world becomes even worse without it. Under our current regime, very few people from underprivileged backgrounds can get into college and advance, but without the SAT, almost no one would be doing so.
Also it's unclear the kids doing well (better than everyone else, I imagine) are the ones who'd most benefit from special treatment, versus the struggling kids.
No kid should get special treatment; every kid should get treatment appropriate to their level. It's not even strictly speaking a matter of resources: you don't need fancy equipment to teach a more advanced class. It does mean you need more teachers and a larger school, so that people can be placed into classes of roughly equal levels of ability.
Or put differently: for this suggestion we can't pay. And we choose not to pay for even mild improvements to the current situation.