The extent to which having a high-quality teacher in the classroom amplifies that gain is unknown. We don't know the answer to that. There's no question about it. You can get a greater return more quickly in terms of student achievement by reducing classes. I'm not against teacher training - I work in a school of education.
Teaching is a very complex, context-related activity.
Further, some of the most profound effects of good teaching may not be identifiable using annual, standardized test data. Improving teacher quality is a very complex, long-term undertaking. I sometimes feel that, at least on the part of some people, the argument about teacher quality represents more a political tactic to blunt the movement to reduce class size, than a commitment to improving teacher quality. SAGE is often used as a shorthand for small classes. SAGE has its origins in State Superintendent John Benson's Urban Initiative Task Force.