Affirmative action has no place at that end of the pipe. You need to be addressing the root cause of lower inputs for minority students instead of pretending that admitting them has the same rate of success that admitting higher scoring students does. As a society we seem to think it's on everyone to make a difference at every level but unless the earliest stages have bought into that it's way less effective than it…
Additionally, we would need to provide various ways for kids to spend more of their time at school (how else are you going to lessen parents' influence)?
And then, after holding on to such an investment for 18 years (a bit more), we will begin to see results?
Aside from the cost, because this would mean 300% raises, minimum, for teaching staff, this would be a social bloodbath, and the education sector will fight anything that goes 1% in that direction to the death.