The strangers are embarrassed and unwilling to sing together. He stands awkwardly to one side, she with her arms folded. But things soon warm up and before long the two are belting it out like Britney Spears and Justin Timberlake. After the song is over the two part company, little knowing that fate will soon bring them together again as Gabriella enrolls at Troy’s school and turns his world upside down.

Will Troy shake off the disapproval of his sporty friends and sing in the high-school musical? Will uptight Gabriella let her hair down and get together with Troy? And will the pair see through the dastardly plans their rivals hatch to get them out of the school play? If you don’t care, prepare for a long summer. The story might seem hackneyed to anyone over 20, but High School Musical has caught the imagination of an army of American kids and is about to sweep the world.

Produced by Disney and released last New Year’s Eve, High School Musical has been seen by 36.5m people, including 11.5m adults. The soundtrack has gone triple platinum and has been in the top 10 of the Billboard music charts for 19 of the 20 weeks since its release. The DVD was released last week and sold more than 1.5m copies in the first seven days, making it the fastest-selling new release of the week.

Not since Grease or West Side Story has a musical had such impact. A full-scale Bollywood version is in the planning stages as is a Japanese version. The movie is released in Australia this month and in September its cast will descend on London for a full-scale celebrity-packed premiere.

But this is no big-budget Hollywood movie. High School Musical was made for television for the modest price of $4.2m (£2.2m). It has no stars. Even Disney seems taken aback by the sheer scale of its success.

“We always thought we had made a good movie,” said Gary Marsh, president of Disney Channel Worldwide. “But what is surprising to me is that what started out as a movie has become a cultural moment.”

He said children had bought the CD in huge numbers and had made 1.5m downloads from iTunes.

But they have also made their own versions of the musical. On YouTube.com, the popular website where people post their videos, fans have been putting up their own High School Musical videos. They include children re-enacting the dances, clips with new songs played over them, and even a chihuahua supposedly singing one of the songs.

In America, Disney has released a version for schools that want to stage their own High School Musical. Marsh said they had so far had 40,000 inquiries. He said the movie had become a hit in part because its themes were “universal”. “The notion of fitting in socially or culturally doesn’t end with high school. Being able to declare who you are is part of what it means to be a human being.”

But he said High School Musical might also signal a newer trend in society: the financial arrival of the “tween”. Depending on who is doing the counting, “tweenagers” vary between the ages of 7 and 14.

According to Wendy Liebmann, president of the analyst WSL Strategic Retail and author of How America Shops, tweenage “is a state of mind as much as an age”. But what is certain is the growing buying power of pre-teen children.

Tweenagers have been estimated to be a market worth $36 billion but their real power comes from their influence over their parents. According to WSL research, 61% of adults will “occasionally or always” defer to their tweenage children when buying music or DVDs. They also take their advice on everything from electronic devices and shampoo to pet food.

“When kids as young as eight are deciding where their parents shop, it is very important for companies to reach them,” she said.

In one of the weeks when High School Musical topped the American album charts, the other two slots were filled by the soundtrack for the kids’ film Curious George and by Kidz Bop, a compilation of children singing the latest chart hits.

“There has been a lot of talk about the tweenage market but mainly in terms of clothing and entertainment. Nobody thought they could move the music business,” said Marsh.

The big music labels are now courting the stars of High School Musical in the hope of discovering the next Britney or Justin, both former Disney child stars.

William Strauss, author of Millennials and the Pop Culture, said a number of different cultural phenomena had come together to make a monster hit of High School Musical, including a renaissance in youth theatre in America and Canada.

“Just look at what they have been watching,” said Strauss. Raised on DVDs of musicals such as The Little Mermaid, The Lion King and Beauty and the Beast, tweenagers had grown to love musicals, he said, pointing to the success of the musicals Wicked and The 25th Annual Putnam County Spelling Bee, a musical based on a high-school spelling contest.

But the film’s real success has stemmed from its ability to target tweenagers’ parents as well as the children, he said.

Many tweenagers are the children of Generation X , people born after 1961. Generation X parents are technologically sophisticated and happy to let their children download content from iTunes. They also want to watch films with their children but are concerned that the content is safe.

“The blandness of High School Musical makes it ideal for tweenagers and their parents,” said Strauss. “The new-generation children have a powerful urge to share, both with their friends and their parents.”

When they find something they like and they get their parents’ approval, tweenagers hit the internet and their mobile phones to spread the message around the world. Strauss pointed to the success of Harry Potter as another example of when tweenagers’ and parents’ interests coincide to create a global phenomenon.

“If you find something tweenagers approve of, and their parents approve of, you can get the product very right,” he said.

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