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.