I'd like to put some finer shadings on Joseph's recent post. I think I speak for most former and current teachers when I say that my favorite part of teaching was teaching -- explaining concepts, helping students working through problems, convincing them that they can not only do math, they can actually be good at it.
Helping a student who has shown an interest in learning always takes a high priority, but at some point something has to give. When I was teaching I routinely worked fifty plus hours a week when you count grading and making lesson plans. Add being on call ten or more hours a week and you will eventually have to make a choice. You can blow off the kids, you can blow off the grading and lesson plans or you can find out how long you can go without seeing your family or getting over six hours sleep.
As far as I can tell, the underlying assumption of the KIPP business model is an unlimited supply of Erin Gruwells. In case you've forgotten, Ms. Gruwell was the... let's say, inspiration for the character played by Hillary Swank in the movie Freedom Writers. Ms. Gruwell has managed an exceptional career but as a teacher, she followed a very common trajectory. She came into the field with high expectations and tremendous energy, worked incredibly long hours at a fantastic pace, achieved some early successes then bailed in less than five years.
KIPP burns through these teachers at an extraordinary rate ("Annual teacher turnover rates have ranged from 18 to 49 percent since 2003-04"). This is possible in large part because there are less than twenty-seven thousand students in the KIPP system. By charter school standards, this is substantial but there are over sixty-million K through 12 children in the U.S.
A number of people think KIPP is a scalable model for the nation. Many of these people hold advanced degrees from some of America's most prestigious universities. If reformers are looking for an indictment of our educational system, I can't think of a more damning one.