40 under 40

Jeff Nelson was a classroom teacher at South Shore's O'Keeffe Elementary School in 2006 when he read a front-page story with a dismal stat: Only 6 percent of Chicago Public Schools freshmen would go on to earn a four-year college degree. Which of his 32 sixth-graders would be the lucky two?

The news sent Nelson—at 24 a Teach for America star in line to be the rigorous recruitment program's area director—spiraling into pre-midlife depression. "That was a crucible moment," he says, one that triggered soul-searching for a bigger calling...