To say that teacher quality is the most important variable in education is at best ignorant and at worst a lie.
This isn't the same as saying teacher quality doesn't matter -- it really does.
Diane Ravitch puts it this way:
Since Arne Duncan became Secretary of Education and unleashed the Race to the Top, almost every state has adopted laws to evaluate teachers by the test scores of their students. Most teachers know that this is unfair because the factors that have the greatest influence on students’ test scores are not within the control of teachers. Reformers tell us that teachers are the most important influence within the school on student scores, and that is right. But the teacher contribution to scores is dwarfed by the influence of family and other out of school factors.
