I wrote elsewhere that bad teaching is more likely a leadership fault rather than a teacher's. I do believe in individual responsibility, and bad teaching may indeed be the result of a bad teacher. Equally, it may be the result of other factors, and a good leader will also consider these when making his/her evaluation.
I grew up with the phrase, "horses for courses". The sense of this aphorism is that some horses run better on grass, some on sand, and some on cinders. The same is true of tennis. A claycourt champion does not necessarily do well on grass, and both may struggle on a hardcourt.
Teachers are no different. While some are effective with examination-level classes, when put in front of a low-ability group they struggle and may even resort to Dickensian-style control methods. Preschool specialists are not prepared for adolescents and so on.
Yet too frequently, a teacher is assigned to a group or a level or an age to which s/he is ill-suited and is then held responsible for the inevitable failings. Re-assigning a teacher may remove all concerns. The fault here is one of leadership.
Similarly, every school operates around a set of values, some explicit and some its unwritten culture ("the way we do things around here"). A teacher may be hired due to an impressive resume, or to fill a specific gap such as AP History or Middle School Basketball. However, when s/he arrives, a values-gap becomes apparent and conflicts ensue. This teacher may be highly effective in another setting. Here, s/he is is misplaced and although the new hire is typically blamed, the fault here is with the hiring. Again, it is one of leadership.
Teaching and learning hinge on finding a "good fit", and on coaching and mentoring a colleague into fitting well. A "bad teacher" more often than not reflects the quality of the school's leadership.