I have kids doing scut work because I can't just excuse them. They'll get good grades on the test. They "got it" seconds after I said it. So they have to sit there and be bored instead of trying to find useful things to do. I get it.
But it's disaster for a lot of low-achievers, who need to have everything spoon fed and take lots of little baby steps. It's bad enough, having a kid take a test and fail it after he's not done any of the work that's intended to build mastery. Then he goes and turns in the work late.
Let's say he passes the test when he takes it several weeks later, but because he didn't know that content he failed the next test. He's running weeks behind. The semester ends and because his brain came back from summer vacation a month after his body did and nobody was allowed to kick his butt to get his brain moving, he fails.
And it really doesn't work for kids playing school. They turn in assignments without mastery, they take tests and fail, and in the end they're not going to care about Algebra II or physics, Beowulf doesn't do it for them. They're going to be farmers or welders, construction workers or truckers. They've been playing school for so long--not taking it seriously, but focusing just on checking off the boxes as they turn in assignments--that they can't do the work. Algebra II? Heck, they can't give a good answer to 9 x 8 = _____ without a calculator.
Can't separate them all out--GT kids might be separated if they reach critical mass, but unless you offer different classes and content for the high and low achievers you can't track or ability group.