There's been a lot of attention paid over the years to the issue of teacher turnover rates, under the two assumptions that teacher turnover rates are really high and that high turnover rates is a bad thing for schools--especially for schools serving poor and minority students.

Not necessarily. In a new paper from Erik Hanushek and Steven Rivkin, they assert that teacher turnover on average really isn't much higher than turnover in other professions and that turnover has been given a bad rap. Steady job turnover is generally viewed as a strength of the American job market (versus more inflexible European markets) for both employer and employee.

Regardless of what may be the national average for the profession, the large Texas urban school district explored in this study has a turnover rate that no profession could possibly embrace: literally 30 percent of a school's staff typically turns over each year, either to leave the profession altogether (12 percent), switch school districts (7 percent) or just switch schools (11 percent).

It gets interesting when examining the average effectiveness of those teachers who chose to leave. The "leavers" turn out to be markedly less effective than the "stayers", about the same difference in the average effectiveness of a first-year teacher and second-year teacher. Because the leavers are generally so much worse than the stayers, it doesn't even matter if you replace a leaver with a rookie. (And when the leavers go off to other schools, they generally don't do any better in the new school than they did in the old.)

The exception to this trend is first-year teachers. The better ones switch schools after a year, probably because they are quicker to high-tail out of a bad school.

Why do the less effective teachers leave more often? Given tenure and collective bargaining laws, Hanushek and Rivkin don't believe that it is the schools making those decisions, but teachers themselves. But it may be that principals are better than we think at using informal mechanisms to get their less effective teachers out of the building.

But they are not effective enough to serve as good teacher quality strategy, caution the researchers. Yes, less effective teachers routinely go, but if you look at the teachers who stay, there is still tremendous variation in quality within the school, with evidence that some of the worst are still at their desks.