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I know this is old territory but in case anyone missed this finding (reported by EPI):
"A study designed to test this question used VAM methods to assign effects to teachers after controlling for other factors, but applied the model backwards to see if credible results were obtained. Surprisingly, it found that students' fifth grade teachers were good predictors of their fourth grade test scores. Inasmuch as a student's later fifth grade teacher cannot possibly have influenced that student's fourth grade performance, this curious result can only mean that VAM results are based on factors other than teachers' actual effectiveness."
EPI is the Economic Policy Institute. They may have an axe to grind; I'm not familiar with this group.
VAM is "Value-Added Modeling". The idea is simple and meritorious: find out how much more students learned with teacher A than with teacher B, by using student test scores. Evidently it's not so simple.