Everyone knows that our nation's public schools are in a state of crisis. (Everyone also knows that the public school their individual child goes to is doing a good job, but they assume that there must be a problem with all the others because they keep hearing about "the crisis in our nation's public schools". But that's a whole other post.) But nobody seems to know what to do about it. One of the most popular suggestions involves "merit pay increases" for teachers whose students do well in school.
First and foremost, this is fundamentally wrong-headed. It betrays a lot of the unintentional biases of the people complaining about our nation's education that they think it can be solved by incentive pay; they're assuming that teachers don't really care about the kids, so long as they get a good paycheck out of the deal, and that they only way to motivate them is with a few extra bucks if they make the kid smarter. News flash to everyone involved: Nobody becomes a teacher for the money. The job is thankless enough that you only do it if you really, genuinely like educating kids.
Second, this rests on a false assumption; namely, that teachers can control the learning ability of their students. A host of other factors, from parental involvement to socio-economic background to plain old innate intelligence, helps or hinders a child's education. Doling out merit pay to teachers would be like giving incentive pay to someone for assembling a machine within a certain time frame, then pointing them to a pile of random car parts and saying, "Get to it."
And third, this violates one of the most fundamental and basic rules of doling out money, which is "Never put incentive pay in the hands of someone who controls the conditions of it being released." As mentioned, teachers don't make a ton of money. Telling them that they can have extra cash if they're lenient graders or give the children unethical amounts of assistance on standardized tests is putting a lot of temptation in the way of people who might be in some serious need of money. While most teachers don't do the job for the money, it's Rule Number One: Don't put temptation in people's way, or they might just take it.
Of course, you could take the time, effort, and expense to enforce ethical guidelines and make sure teachers can't cheat...but honestly, you could also just hire more teachers, since the clearest correlation has always been between class size and academic success. But since the whole "crisis" is a thinly-disguised attempt to get rid of the teachers' union to begin with, it's not likely that anyone will propose a solution that involves hiring extra ones.