Rowan et. al., 2002. That research found that:
"In review of this literature, Scheerens and Bosker found that when student achievement was measured at a single point in time... about 15-20% of the variance in student achievement lies among schools, another 15-20% lies among classrooms within schools, and the remaining 60-70% of variance lies among students."Other studies put the effect of teachers much lower, even under 10 percent. Even if you take the highest number for classroom effect - 20 percent - and assign the entire classroom effect to its teacher, then assume a high number of teachers are failures, i.e. 15 percent, that still renders bad teachers only three percent of the overall effect on student achievement (15 percent of 20 percent). The pie graph above shows the effects of students (60-70 percent), schools (15-20 percent), classrooms - good teachers (85 percent of 20 percent), and "bad" teachers (15 percent of 20 percent) on achievement.
If so-called "bad teachers" represent only three percent of achievement impact, and students themselves represent 65 percent, why does the education deform movement focus on the three percent and not the 65 percent? Because it's easier and politically efficient. Focusing on teachers to improve educational outcomes is like trying to squeeze blood from a rock. Conversely, focusing on the socio-economic status of students and their families represents fertile ground for improvement seeing as how it represents such a large chunk of responsibility for achievement.
*Hanushek et al. 1998; Rockoff 2003; Goldhaber et al. 1999; Rowan et al. 2002; Nye et al. 2004