A recent survey by a private organization, the National Center for Education Information, reports that 40 percent of the current public school teaching force expects not to be teaching five years from now. The study, based on a survey of 1,028 randomly selected teachers (with a response rate of 48%), found that most teachers planning to leave the profession are simply due to retire.

Ironically, the rising popularity of alternate route programs in the last two decades may end up contributing to the pending surge in retirements. Even though these teachers may have arrived late in the game, starting to teach in their thirties or forties as a second career, many of them will still elect to go for an on-time finish.

On a related note, a recent study from the Alliance for Excellent Education tried to fix a price on teacher turnovers, pegging what it terms a 'conservative estimate' of the annual nationwide cost of replacing teachers at $2.2 billion. AEE based its calculation on a Department of Labor estimate that attrition costs employers 30 percent of a departing employee's annual salary. However, the relevance of that figure to teaching may be suspect, given that districts typically--and unfortunately--invest relatively little in recruitment and training--far less than their counterparts in private industry, anyway.

Also, pegging an accurate estimate has to account for the fact that school districts tend to devote the same amount of money to recruitment and staffing of their human resources offices whether they end up hiring 400 or 800 teachers, give or take a few temporary employees hired in a crunch to answer the phone. The amount of money going to mentoring also tends to be fixed, either spread comfortably across 100 new teachers or squeezed across 300.

Though coming up with a reasonable estimate of cost has its political benefits, the real price of the revolving door--and the consequence that school boards ought to be persuaded by--is its deleterious impact on student achievement.