Nicholas Kristof used his column Sunday to tell readers about how an exceptional teacher, Mildred Grady, had made a huge difference in the life of a young African American boy. According to Kristof, Ms. Grady saw the boy, a known trouble-maker, steal a book from the library. Instead of turning him in, she bought several other books by the same author, which the boy subsequently stole from the library and read. As a result he became attached to reading. He went to college and then law school and went on to become a judge.

Kristof uses the example to explain the importance of good teachers. He argues that we need regular evaluations, with the teachers who score well getting big pay increases, and those who score poorly getting fired.

However, we do not know how Ms. Grady would have performed on the evaluations advocated by Kristof. It's possible that she would not have done very well under this system. That is especially the case if other less conscientious teachers focused on teaching to the exam, while she spent more effort trying to make an impact on the lives of her students.

It is entirely possible that Ms. Grady would not be one of the teachers rewarded under the system advocated by Kristof. In fact, such a system of evaluation could even drive dedicated teachers like Ms Grady away from the profession.