A reader, Jill Koyama, calls attention to an important topic:

I actually conducted a 3-year study of private tutoring companies in NYC. Here is the link to my book, Making Failure Pay: For-Profit Tutoring, High-Stakes Testing, and Public Schools, published in 2010 by the University of Chicago Press:

http://press.uchicago.edu/ucp/books/book/chicago/M/bo8917055.html