Farr grew up in a family of teachers in central Texas. When he graduated from the University of Texas, in 1993, he had a philosophy degree and an acceptance letter to Yale Law School, neither of which felt quite right. So he deferred law school and joined a new, floundering outfit, Teach for America.

After a little more than a month of somewhat uneven training, Farr walked into Donna High School in the Rio Grande Valley in Texas—a place he’d never been. Many of the three dozen kids in his classroom were the children of migrant workers; they would disappear for weeks at a time as their families followed the harvests.

Talking to Farr about those two years feels a little like talking to a war veteran. You and he both know that you can never understand what it was like, and the clichés come marching in. “It was the hardest, proudest, all of that,” he says, his voice drifting away. Then: “I was not the teacher I want our teachers to be.”

Farr lived with three other Teach for America teachers, in a house that had been confiscated by U.S. Marshals in a drug raid. He taught English and English as a Second Language. Texas required that students pass a standardized test before they graduate, and as test day approached, Farr felt a mixture of anxiety and resentment.

About a month afterward, he got the news: 76 percent of his students had passed; 24 percent were told they didn’t yet have the skills to graduate. Even though many were only sophomores, some of them dropped out as a result. The principal congratulated him on his scores, but Farr cried into his pillow that night. “Some of those kids did not pass because I was not as effective as I needed to be.”

After his two years were up, Farr went to law school, as planned. He came back to Teach for America in 2001—this time in charge of training and support. By then, the organization’s founder, Wendy Kopp, had begun to notice something puzzling when she visited classrooms: many Teach for America teachers were doing good work. But a small number were getting phenomenal results—and it was not clear why.

Farr was tasked with finding out. Starting in 2002, Teach for America began using student test-score progress data to put teachers into one of three categories: those who move their students one and a half or more years ahead in one year; those who achieve one to one and a half years of growth; and those who yield less than one year of gains. In the beginning, reliable data was hard to come by, and many teachers could not be put into any category. Moreover, the data could never capture the entire story of a teacher’s impact, Farr acknowledges. But in desperately failing schools, where most kids lack basic skills, the only way to bushwhack a path out of the darkness is with a good, solid measuring stick.

As Teach for America began to identify exceptional teachers using this data, Farr began to watch them. He observed their classes, read their lesson plans, and talked to them about their teaching methods and beliefs. He and his colleagues surveyed Teach for America teachers at least four times a year to find out what they were doing and what kinds of training had helped them the most.