What Teachers Can't Control
Diane Ravitch argues that "value-added" ratings of teachers based on test scores badly underestimate the impact of out-of-school factors on students' performance from year to year:
Certainly teachers are very important - the most important cause of student success within the school. But scholars agree that factors outside the school are even more important than the teacher, especially family income. Most economists estimate that teachers are responsible for about 10% to 20% of the variation in student scores, and that outside-the-school factors influence about 60% of student learning gains.
In other words, teachers apparently aren't all-powerful. Who knew?