NEW YORK (AP) -- The door to a Hunter College lecture hall opens, and in steps Madonna. There's no tweed for this professor-for-a-day; she wears a black dress and form-fitting boots that stretch to her knees.She's the latest participant in "Stand In," one of MTV Networks' hottest features, particularly given its brevity and relative lack of visibility.The MTVu network, a spinoff seen primarily on college campuses, invites celebrities to be surprise lecturers. Since Jesse Jackson inaugurated the series in January 2004, "Stand In" has featured Bill Gates, Shimon Peres, Tom Wolfe, Kanye West, Ashley Judd, Russell Simmons, Snoop Dogg, Sen. John McCain and Sting."It brings the class to life in a way that few would ever imagine," said Stephen Friedman, MTVu's general manager.MTVu had envisioned a series where colleges would compete to hear a celebrity speak. But that proved too time-consuming to organize and when its second speaker, Marilyn Manson, nailed his appearance at Temple University, MTVu knew it had a better format.Manson walked into a class on art and politics in full makeup, writing "Mr. Manson" on the blackboard and setting down a bottle of absinthe before the startled students. He then led a discussion on the role of provocative art in society, saying "art to me is a question mark. I don't think it should ever be an answer."Gates, the one-time computer geek turned world's richest man, surprised a University of Wisconsin class on introduction to programming. McCain requested a visit to his alma mater, the U.S. Naval Academy, to talk politics.The students' reaction is key; most episodes someone with mouth agape at who has just walked into their sleepy classroom.Participating colleges and MTVu try to keep the secret by telling fibs to students who may wonder about the cameras when they show up to class.At Hunter last week, a film class was told it was screening Madonna's new documentary, "I'm Going to Tell You a Secret," and discussing it with the film's director. With an endless stream of adults walking in and out of the room during the movie, smart students figured out what was happening."Since there were security guards all lined up I figured she was coming," said Pinar Noorata, a junior film major. "That was kind of a dead giveaway. But I think everybody was still surprised. It was kind of surreal."As the students stood and applauded Madonna, about a half-dozen pointed their cell phone cameras in her direction so their friends would believe them later.They lobbed mostly softball questions about the film, Madonna's interest in kabbalah and her two-decade journey through different musical incarnations."I don't feel like I'm trying on personas," she said. "What I always hope to do is change and evolve. I have no regrets because that's life and life is about change."She looked back at students who weren't much younger than her when she made her journey from her home in Michigan to New York City, hoping to make it first as a dancer, then as a singer. She counseled self-confidence and tenacity."The biggest mistake that any of us can make is to believe what other people say about us," she said.Creating excitementFor the celebrities, the appearances offer a dose of hero worship in a carefully controlled environment, before a youthful audience many of them need to court. The white-suited Wolfe seemed genuinely juiced to stand before a class that was studying one of his novels.There's also the chance to promote a pet cause, like on Thursday when actress Cameron Diaz jolted awake an 8 a.m. Stanford University civil engineering class. She appeared with architect William McDonough to talk about building designs that protect the environment."I was expecting like 10 kids to show up," Diaz told The Associated Press later. "It's exciting. A few of the kids came up afterward and said, 'This is so great, this is something I'll remember.' Hopefully, it's something they'll be thinking about when they are sitting down trying to create."Sting brought his band to an advanced musical composition class at the University of Illinois-Chicago, offering one thrilled student the chance to add a flute solo to "Every Breath You Take.""To make it more meaningful, you really have to have the right class," Friedman said. "What makes this work is the setting of the right person in the right class."Seeing much of the stand-ins is a challenge, though. MTVu leaves most of the appearances on the cutting-room floor, boiling each down to a four-minute sound bite. MTVu is available only on television systems in dorms and dining halls at 730 college campuses, although this fall it became the first MTV Network streamed continuously on the Web. Past appearances are archived and can be viewed through the station's Web site.MTV is considering giving "Stand In" some exposure on the main network, Friedman said, and is also mulling making extended versions of the appearances available on the Internet.For Hunter College senior Ruomi Lee-Hampel, it turned out to be one class definitely not worth skipping."Hearing a director speak about his work was my purpose in coming," he said. "It was just an added bonus to see Madonna in fishnets."Copyright 2005 The Associated Press. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.