The 2011 TIME 100

School can be an intimidating place, one in which competitiveness and power cliques can sometimes turn downright nasty — and not just for the kids. Michelle Rhee, 41, knows that as well as anyone — and has handled it better than most. The former chancellor of the Washington, D.C., school system and the founder of the Students First advocacy group, Rhee has distinguished herself by her single-mindedness and her dedication to kids.

Anyone who works in the political world quickly learns that personalities and competing agendas have a way of devouring people's best intentions. Not Rhee's. She set a goal to improve the lot of the nation's students, and she has stuck to that. And she paid dearly for it, stepping down from her D.C. post in 2010 after Mayor Adrian Fenty lost his bid for re-election, a public rejection that some saw as a repudiation of the tough steps Rhee took to raise the standards of the city's public schools. Subsequently, she shunned any high-salary job offers that resulted from her high-profile tenure and instead founded her organization. "Putting kids first" could be a pithy slogan. (For many it is.) For Rhee, it's a lifetime commitment.

Guggenheim is the director of Waiting for "Superman"