At my school, a small public high school in Brooklyn, New York, well over half of the teachers at the school are Teaching Fellows, and, at least in the three years I have been at the school, the longest any of us has stayed (yet) is three years. A few of us are starting our fourth.
And this sucks for our students. I mean, it really, really sucks. It sucks to come back to school and have to have yet another first-year-teacher as a teacher. It sucks to have six different advisory teachers in four years (the case with my old advisory). It sucks to have no continuity from year to year. It sucks for the ninth grade math teacher you really liked to disappear by the time you are in eleventh grade and wanted to ask for some extra help before the PSATs. It sucks to slowly get the impression that teaching anywhere else, or doing anything else for a job is better than staying here and working with you. It sucks to get abandoned year after year after year by young, enthusiastic teachers who saw teaching in the inner city as something great to put on that law school application.