There's a new policy brief showing that Teach for America teachers -- recent college grads plucked from elite schools -- don't do better than credentialed teachers when it comes to student test scores. And the reason is pretty straightforward: They're inexperienced and generally leave before they develop the skills to be effective educators. Before they're thrown in a classroom, TFAers get five weeks of training and over half leave upon completing their two-year commitment; 80 percent leave after three years.