The same as in any other job: we rely on the honesty, skill, and self-interest of their bosses and peers. Honest and accurate evalution of performance is the signal, stupidity and bias is the noise. Somehow, though it's pretty frickin' amazing when you think about it, people who are chosen, guided, and evaluated by biased, flawed, mostly stupid human beings manage to treat disease, build bridges, publish newspapers, maintain sewage systems, and do everything else required to keep society going.

I mean, I work in a vast and faceless corporate bureaucracy driven by the most idiotic politics imaginable, but when I think back to my favorite teachers from high school, they got a raw deal compared to me. Even though I got laid off my first job because of the dot-com bust. Even though I left my last job because of an abusive boss. Even though my review process is cookie-cutter and somewhat divorced from reality. It would have been better than having no opportunity to differentiate themselves from the stupid, lazy, coasting teachers who climbed right up the pay scale with them. Heroic, good, average, bad, and sleepwalking, they all have the same job title and same pay all the way to retirement -- it's no wonder teaching isn't respected as a profession.