The achievement gap between rich and poor students in the United States has confounded educators for decades. Wendy Kopp posed a novel remedy in her Princeton senior thesis: persuading hundreds of graduates of top US universities to spend two years teaching disadvantaged students before starting careers in investment banking, medicine, and the like or continuing in education. Teach For America, as her nonprofit organization came to be known, would train prospective teachers intensively over the summer and then place them in the classrooms of low-income communities across the country. Kopp believed that in addition to boosting the students’ achievement by enlisting bright, energetic teachers for schools that find it hard to attract them, the organization would turn the recruits into lifelong advocates of educational reform.

That was in 1989. Within a year, Kopp had created a skeletal group of recent graduates, raised $2.5 million, and persuaded 2,500 college seniors from universities such as Harvard, Spelman, the University of Michigan, and Yale to apply for 500 teaching positions.¹ Six school districts, from New York City to rural North Carolina, eagerly awaited the new teachers. Before long, news organizations such as the New York Times and the Public Broadcasting System...