Gothamschools has some good figures and graphs showing trends in the distribution of "unsatisfactory" ratings in New York City. It's worth checking out the full story, but here's the jist:

City principals rated more teachers unsatisfactory this year than they have since at least 2005, suggesting that the Bloomberg administration’s efforts to escort more struggling teachers out of the system may be bearing some fruit.

Principals gave the scarlet-letter rating to 1,554 teachers this year, up from 981 in the 2005-2006 school year, data provided by the city Department of Education show. Both the number and percentage of teachers rated unsatisfactory rose during that period, and the rise occurred for both tenured and non-tenured teachers, city figures show.

Even with the rise, the percentage of teachers rated unsatisfactory remains low. About 2% of teachers, both tenured and without tenure, received what teachers call “U” ratings this year.