They are charged with turning these little rugrats into educated civil human beings. To the extent the rugrat morphs into a responsible accountable "successful" human being then that will be the yardstick of the efficacy of the teacher. Your kid fails you sue the teacher. Your kid succeeds you don't sue the teacher. You have no intention of rewarding teachers for doing their job well. It is their job to do well. You only punish them for not doing the job.
Sounds fair. Why any normal sensible rational logical human being would subject himself or herself to the wrath of lazy demanding insufferably hypocritical parents is beyond incomprehensible. But they do it because well they love teaching and they love "their kids". So much so that they even spend THEIR OWN MONEY to provide things for "their kids" that the school budget won't provide. So for teachers it isn't sink or swim. It is swim OR ELSE. That should satisfy all you anti-teacher folks out there. Get your pound of flesh. Get your satisfaction. SIGH.