The problem is that parents make a stink when their special little snowflake is given a bad grade, or sent to the principal, or whatever. But parents don't make a stink when their kid doesn't learn, because the teacher is too busy dancing around the kids that s/he can't do anything about because their parents would make a stink.

One disruptive kid can prevent 20 kids from learning. Look, the kid may have reason. His parents abandoned him, he's hungry, whatever. And it's not fair to just drop him because his parents did. But it's also not fair to let him keep everyone else from learning.