Implicit in a lot of the report was that the "Irreplaceables," or high-performing teachers, are the young, motivated ones while the low-performing teachers are the older, experienced ones. On page 14 it cites issues like, "compensation systems that pay the least effective teachers more than the most effective" as major factors in the high-performing teachers leaving. Certainly this was written by people who know that seniority is usually the basis for salary, therefore this line is directly implying that experienced teachers are less effective overall. I understand that motivation is an important drive for an effective teacher, but a first-year Teach For America student is almost definitely not going to be as skilled as a 10-year experienced one.
Moreover, the conclusions drawn in this report are based on only one year of data. I am by no means an expert on educational research but knowing that measurement of an effective teacher can change quite a bit over two years means they should not draw any major conclusions from this information.
A related study (which was in the peer-reviewed journal Educational Evaluation and Policy Analysis in 2004) based on four years of randomized classrooms and teachers pointed out that, "in low-SES [socioeconomic status] schools, it matters more which teacher a child receives than it does in high-SES schools." According to this research, teacher effects are different based on the socioeconomic status of the neighborhood in which they teach. If this is true then it is truly difficult to measure teacher effectiveness overall (something that has been pointed out a lot recently) and we should really focus more on poverty's effects on schools.
The report does point out something quite important: "Less than 30 percent of Irreplaceables who plan to leave their school do so for personal reasons beyond their school's control, and principals hold significant sway over the decisions of the other 70 percent" (p.16). If these good teachers were given regular feedback, given time to identify areas of development, put in charge of something, or provided with additional resources, potentially they would stick around.
Additionally, if there were alternative pathways for career advancement, some teachers would stick around in the classroom longer. Potentially these individuals could share their time between teaching children directly or leading parts of the school. A lot of this I've mentioned earlier in posts about the RESPECT project coming out of the US Department of Education.
I think this report touches on important points but in a very politicized (and unscientific) way that is not really helpful in the long run.