Of course...students are widgets....they all respond exactly the same to instruction.
No, some are smarter than others.
I'm not opposed, by the way, to teachers being expected to be at least competent, if not great. Do not mistake me. It's not okay to have sub-adequate teachers in our classrooms. But I don't think you measure this by output on standardized tests, because you are talking in some cases about young children coming from a variety of backgrounds who, in some situations, just CANNOT learn. No matter what the teacher does or does not do, or how competent the teacher is.
There are other methods of judging teacher competency that, when blended with student standardized test achievement over time, would give a better picture. But not a snapshot of student test achievement one year and then one year and then one year. Because there are too many learning factors at play over which the teacher has no control.