Let’s think about it from the principal’s perspective: it’s October. If you put a teacher on a 90 Day Plan starting Nov 1, she won’t be out until early spring. In the mean time, how effective do you think that teacher will be at teaching, especially since she can safely assume no one is looking for her to improve and get out of termination? And at the end of January, who will you get to fill that spot? Just how many excellent teachers do we think are hanging out in January, looking for a job (with the possible exception of Dr. Seibens)? Might not your students be better off with their (even low-performing) teacher and some instructional support?
I suspect that Michelle Rhee has either built in some extra incentives for principals to put teachers on the 90 plan, or just put the fear of God in them about their own jobs unless they do it.