Pamela's Analysis (histogram/distribution plots of minimum of first/last year in school data):

http://www.nctaf.org/resources/news/press_releases/CTT.htm: 2007 national data: 15% national, 20% urban
http://nces.ed.gov/pubs2010/2010353.pdf These percentages also reported by the National Center for Educational Statistics
p.4: Comparison chart of typical turnover rates by state:

from http://www.ncsu.edu/mentorjunction/pages/Research.htm