[… The] statistics that do exist on teacher performance are problematic for the bad-teacher hypothesis of the reformers. To counteract some of the problems of cross-teacher comparisons, reformers have wisely adopted value-added methods of evaluation which measure how much a student has improved while being instructed by a specific teacher. The value-added methods avoid the year-to-year variability inherent in measurements of the absolute skill-level of students. If teachers do really have a significant impact on students, you would expect the improvement statistics of their students to be roughly the same year-to-year. Their students’ absolute skill-level might vary based on the skill-level they had at the beginning of the course, but their level of improvement should be same.
This, however, is not what happens. An Economic Policy Institute paper notes that
One study found that across five large urban districts, among teachers who were ranked in the top 20% of effectiveness in the first year, fewer than a third were in that top group the next year, and another third moved all the way down to the bottom 40%. Another found that teachers’ effectiveness ratings in one year could only predict from 4% to 16% of the variation in such ratings in the following year. Thus, a teacher who appears to be very ineffective in one year might have a dramatically different result the following year. The same dramatic fluctuations were found for teachers ranked at the bottom in the first year of analysis. This runs counter to most people’s notions that the true quality of a teacher is likely to change very little over time and raises questions about whether what is measured is largely a “teacher effect” or the effect of a wide variety of other factors.
Unless teachers’ quality radically changes year to year — something that would still be problematic for the reformers — the only thing that can account for these variations are things outside of the classroom.