To put everything back within the analogy klash describes--which was quite vivid, actually--I see what klash is saying, but Orange Julius is correct when he says that public schools generally don't try to make all kids the same brick.

To sprinkle some math on, I think it'd be more correct to say that schools are able to deal competently with bricks that are within one standard deviation in either direction of being average bricks--you just add varying amounts of cement, and you can still make them all fit in the wall nicely.

The problems arise when you get bricks that are way too small or way too big.  The really small (dumb) bricks aren't worth putting into the wall, and conventional bricklaying logic says to discard them.  Schools are not bricklayers in reality, however, and must deal with the social and ethical consequences of throwing away the small bricks.  Since they can't ethically throw away any bricks, they have to put all the really small bricks in a corner, throw the third-rate cement on them, and try to slap a special wall together.

And the really big (smart) bricks, those require too much cement to get them to fit into the wall.  It also requires restructuring all the other bricks around it to make a nice fit, which slows down the bricklaying.  Since schools can't throw them away, they generally try to either

A) Pretend that the big bricks aren't all that big and hope no one notices, or
B) Let the big bricks go to a more advanced stage of bricklaying before they're emotionally ready (Okay, so the analogy is a little weak here.  Sue me.) to get rid of them.

So, while the shotgun system makes really good, stable, medium-sized brick walls, there's a pile of leftovers that all the politicians are conveniently ignoring.