Fergie teaching Sex Ed? Sounds like the plot of a new reality show—but it was for a good cause. On Feb. 5, the pop star got high marks from 30 10th graders at New York City's Murry Bergtraum High School, where she spoke about AIDS prevention and safe sex. The talk, part of a campaign sponsored by M.A.C Cosmetics, gave Fergie, 32, an opportunity to tell about her own tough years as a young adult (she battled crystal meth addiction) and relationship mistakes: "In high school I cheated on all my boyfriends—whatever!" That risky behavior isn't something she wants young people to emulate. "Do you really want that person who's just going to like you if you have unprotected sex?" she told a female student who complained about boy pressure. "Then they don't like the real you anyway!" Instead, Fergie, who got engaged to Las Vegas star Josh Duhamel in December, wants girls to see that commitment can pay off. "My girlfriends gave me a stack of wedding magazines," she says. "But right now, we have no idea where or when." Meantime, she plans to keep talking to teens. "It's about building self-esteem," she says. "That's what it's going to take for young people to have protected sex. They have to stick to their guns."