That and in addition we have a situation where we protect bad teachers.
Billy has poor marks? He must not try very hard, he's a good kid with lots of potential, but he doesnt apply himself.
*wait no couldnt possibly be the teacher is shit*
Personal story: I'm terrible with english, my english teacher was a bad teacher, I was a 90s student otherwise. He never taught worth shit. He would be 15mins later everyday for the 1hr class. Half the time he wouldnt actually teach. He would simply talk about last night's basketball game or some movie or whatever. On top of that... as an anti-cheating measure we had to write our essays, on a topic he gave during the class, in class. So we were given 30-45mins to write 3-5 page essay. Everyone would fail unless they cheated. If he didnt like you. You would get caught cheating. While if you were his friend... you could cheat at will as he never said crap against you. Infact he would talk to these friends out of class and tell them ahead of time the topic of the essay, they'd write the essay and bring it to class on the day.
So I complain and complain and everyone says it's my fault for not trying hard enough. I go up the chain. Which then gets back. They basically blind the teacher. He has to start giving the tests the other english teacher gives. The same ones. Since he never taught worth a shit. Everyone basically starts getting about 20% on the tests. Which proved how shit a teacher he was. Except he got to imput the marks and decided to give all his friends 90s then basically input everyone else on a curve. Despite that curve i was basically at the top of the curve. I still end up with a terrible mark and no knowledge.
tldr; bad teachers are protected and hurt students.