There are a couple of things going on there. First of all, schools are really designed to teach kids en masse. If you had to teach ONE kid, you'd never set that kid up as we do, where a kid sits down and a teacher talks to them from the front of a room and, after that, gives the student a few minutes of individual attention. The classroom format is designed to maximize the amount of the teacher's time spent teaching while trying to maximize the aggregate learning of the class. But, that means that, for any particular kid, there's a fair bit of the class time when they're not learning at all or learning slower than they are capable of.
You can offset that, some, with differentiated instruction where you put the "smart" kids together -- there, the teacher can go at a faster pace, can provide more challenging work, and so on. But, it still has the same fundamental flaw -- you're optimizing for use of the teacher's time, not the student's.
The other thing is that schools, especially public schools, are bureaucracies that tend to be driven by centralized policy, not by individual decision making at that staff level. Part of that is necessity, but part of it is because administrators don't trust student-facing staff to make good decisions and are also hyper-concerned with liability when student X gets to do something when student Y doesn't. If X and Y are of different races, then there will be a claim that Y was denied BECAUSE of his/her race, even if the decision ultimately made sense for both X and Y.