Parents of children who wind up in the classroom of a bad teacher are annoyed. Some parents might even try to get their child removed from that class.
The quality of a teacher has a lot to do with how a child performs.
I was in school many years ago but vividly recall two shocking teachers at my school in Timaru. One was a maths teacher who didn't speak clearly, leaving us with no idea what he was talking about or what we were supposed to do. He ignored the students' questions, mumbling with his back to us while he wrote on the board. Subsequently he hadn't a clue which students were excelling or falling behind.
Another was an English teacher who was timid and had no control over unruly and noisy students. The class lessons would often be a joke with the noisy kids taking over.
I was no dux, but got good results. In the years I had those two teachers my exam results were average.
Other industries reward their staff with bonuses when they excel. Why shouldn't teachers get this also? If some teachers weren't achieving like their peers, then perhaps it's time for them to consider a different career path. The education of our children is too important.