There seems to be a common temptation to chalk student performance solely up to teachers. This despite the fact that there are plenty of things that are out of teacher's control that strongly affect how well a student performs. Here in Maryland, there have been a bunch of recent articles about an alarmingly high percentage of students coming to school without getting adequate nutrition outside of school hours. This has a huge effect on concentration acquisition of new material. In general, socioeconomic and cultural differences between communities have very strong effects on schools; teaching in a school district with highly involved parents gives a huge boost to test scores vs. districts with largely uninvolved parents. Same for rich vs. poor districts, rural vs. suburban districts, etc.

Also, what makes you think school administrators have some answers that teachers don't when it comes to how to do the job, or have any idea, really, how to fairly and correctly evaluate teacher performance. Similarly, why do the same people who routinely call for teachers to be fired for poor performing classrooms never seem to want to hold administrators and school boards to account for their poor performance? Why do state and local governments who cut education funding year after year get a pass?

I think it's a comforting world view to think that if we could just bust the teacher's unions and start firing people, we'd get real results. But it ignores the fact that the system is broken in may more ways—and in many worse ways—than just the teachers.