I have to disagree with you all- I know all too well from relations who are teachers and experience with own DD that it is VERY difficult to sack bad staff, especially in the teaching profession.
My poor DD started A-level maths with a new teacher that year. She wasn't a massively gifted student, she had gotten her A by sheer hard work (maths comes naturally to some people) but she was desperate to do it. Throughout She would always come home with complaints that she was a little bit odd, even insinuating that she had psychological problems from her mannerisms and odd outbursts- I brushed this off, especially as her marked papers were all A grades.
Met her teacher myself and got oddball vibes, but she told me myself that my daughter was a gifted student and was definitely on track for an A*. DD was pleased at this revelation and started to warm to her, but we had another huge problem when one of the boys in her class came in one day and told the teacher that her marking was way off. He was a very clever boy and had gone home and worked with a revision guide, and they had been taught completely differently to how they were supposed to- resulting in wrong answers. At the moment, I thought this was an exaggeration- and when the teacher started crying, they thought she was just having a bad time at home or something and marked badly because of this. Oh they were all so wrong :(
Exam results came back. DD failed and got a U. A lot of the class failed actually. I went in and complained with all these stories but the teachers seemed to think I was just defending my bad at maths daughter. A lot of other parents complained too, and went in with their kids textbooks (some of those even good at maths explaining why half the stuff was marked wrongly) Teachers felt sorry for us but said they couldn't do anything as this wasn't adequate proof.
This was january exam time so my daughter quickly moved to another course, but those who continued would go on to get awful grades in january. Only when my daughter got her A2 results this year would I find out what happened from DD's language teacher who didn't really mind to tell me as she was leaving anyway.
This teacher had bi-polar disorder, a very bad case of it. She said it was found that she was teaching kids the wrong formulas, wrong settings on the calculator, marking them too highly and predicting them too highly. The staff couldn't sack her as that was her only offence- as well as her illness causing trouble, so they asked her to leave quietly in the new year. And along with her went all of the evidence of shoddy marking.
:( so glad DD didn't continue it, could have really messed up her university application.