As a former principal, teachers and I were often dismayed when the time and effort we put in to help these young people be successful went for naught because they often left after their two-year commitment was completed.
A second problem is that these inexperienced teachers are often placed in inner-city schools where teaching is most difficult. Having rookies as their teachers often compounds the severe challenges faced by our inner-city students.
Sadly, I believe that the high attrition rate in Teach For America is mainly caused by the teach-to-the-test mentality of "reformist" programs such as No Child Left Behind. If I were an intelligent, highly motivated and idealistic recent college graduate, I too would run for greener hills after seeing the very creativity that made me a high-ranking grad being sacrificed to testing.