It’s not uncommon to hear someone inaccurately state that the teacher has the biggest influence on student achievement — period. Of course, the true statement is that — of the in-school factors — teachers have the biggest influence. On top of that, research has shown that over two-thirds of the factors that influence student achievement occur out of school.

To illustrate this, here's a pie chart

Lots of good links ot dem,onstrate the evidence behind this pie chart at the link.

If we're primarily focusing on teacher quality to increase student achievment, we're focusing on the wrong place. It's why corporate education reforms are doomed to failure.