On a recent afternoon while waiting to see Gary Marsh, I found myself in an atrium with a few dozen young actresses there to audition for a Disney Channel Original Movie—DCOM in the company lingo. Most of the girls—impossibly cute, perfectly featured, tirelessly smiling—sat with their mothers, poring over the page of dialogue they were to read during the casting session. A few stood up in order to get more into character, gesticulating toward Cheetah Girls posters as they ran their lines. The stench of collective aspiration in the room was as thick as the smell of peach and strawberry body spray. They all want it so badly, and only a very few can get it. Disney Channel execs are famously finicky about their casting, insisting, for example, that they meet and meticulously appraise the parents of any child they are considering hiring. Today’s callback is for a project starring Demi Lovato and Selena Gomez, tentatively titled Princess Protection Program, about a princess who has to enter the witness protection program. It will be the 76th DCOM and, these nervous young thespians hope, a star-launching vehicle as powerful as the 61st DCOM: High School Musical.

Produced for $4.2 million in 2005, High School Musical has spawned a franchise that is worth, according to some analysts, half a billion dollars. The burst-into-song story of a high-school basketball player who secretly wants to sing in the school play, Musical is the most successful made-for-TV movie, the most successful cable movie, and the most successful television-musical franchise in history. More than 8 million CDs, 8 million DVDs, and 9 million novels have been sold, and it has toured as a sold-out concert and ice show. Nearly 3,000 high schools in the U.S. have staged productions of the show, and local-language versions are being produced in Argentina, Brazil, India, and Mexico. High School Musical 2 has put up similar numbers and sold 3.8 million videogames. High School Musical 3 will begin filming on April 22; prescient Disney execs got it locked down before the writers strike, and it will be ready for release in October. Disney Consumer Products has estimated that retail sales generated by the franchise in 2008 will be more than a billion dollars.

High School Musical has the potential to be Hannah times 10, as the same multi­platform success that has worked for one star could be replicated throughout the huge cast of Musical. And Disney execs are paying close attention. Director Kenny Ortega recalls that before the first High School Musical, there would be one or two corporate types sitting in on develop­ment meetings. For High School Musical 3, development meetings require entire conference rooms, as a dozen suits drop in to hear about Ortega and writer Peter Barsocchini’s vision for expanding the franchise. “Very carefully” is Ortega’s short version of how he will proceed. “This is the last High School Musical as we know it, with these characters. But this is a place, a concept, that will keep on giving.” The new film’s budget exceeds $30 million, and star Zac Efron will reportedly pocket $5 million, more than the entire budget for the first Musical (though Disney says that number is too high). But Ortega is eager to assure me, and perhaps his Disney bosses as well, that he will be introducing new characters, further extending the brand.

Ortega, a onetime member of subversive rock act the Tubes—“I was definitely the most conservative member,” he quickly points out—believes that the simplicity as much as the casting of Musical has made it so enduring. The story is Romeo and Juliet, without the Montagues, Capulets, or Friar Laurence. Or any romance. Or danger. Or tension greater than “Will he make it to rehearsal on time?” The two main characters don’t even kiss—but they do exchange a longing glance.

And kids love it. The secret to the success of this franchise, like so many of Disney’s other tween machines, is that it aims even younger than other pop culture.

The success of the first High School Musical, which aired in January 2006 and has been seen by an estimated 250 million viewers, and High School Musical 2, which essentially duplicated its predecessor’s results, prompted Disney to plan to release High School Musical 3 in theaters. Executives were most likely seduced by the simple multiplication of 18.6 million (the audience for High School Musical 2 when it made its debut in August 2007) by $12 (the price of a movie ticket). No less an authority on starmaking than Simon Cowell has called the Musical franchise the only thing out there that threatens his American Idol for supremacy in the music biz.