There are huge variations in teacher effectiveness. Some teachers are really good at getting their students to learn, and others are not.
Teacher quality matters so much that a student is likely better off in a bad school with a good teacher than in a good school with a bad teacher.
You can find ineffective teachers in good schools and effective teachers in bad schools. But struggling schools tend to have more ineffective and inexperienced teachers.
African-American students may be particularly likely to get ineffective teachers. According to a study of two large school districts in Tennessee, black students were overrepresented in the least effective teachers' classrooms by about 10 percent. They were also under-represented in the most effective teachers' classrooms by 10 percent. This is from a study by researcher William Sanders, who has done a lot of pioneering work about teacher quality.
Students of the most effective teachers make excellent academic progress regardless of their prior achievement levels, while students in the classrooms of the least effective teachers do not progress well.
As the level of teacher effectiveness increases, the lowest–achieving students are the first to benefit.
Having a high–quality teacher throughout elementary school can substantially offset or even eliminate the disadvantage of low socio-economic background. If poor children consistently got great teachers, the achievement gap could disappear.
The effects of bad teaching last. Even if a student eventually gets great teachers, those who had ineffective teachers in the past continue to lag behind their peers. "Although an effective teacher can facilitate excellent academic gain in students during the years in which they are assigned to them," writes researcher William Sanders, "the residual effects of… ineffective teachers were measurable two years later, regardless of the effectiveness of teachers in later grades