Chance brings them together again when she moves to his school. But how can they be together when he’s a jock and she’s a geek? How can they overcome the villains keen to separate them, and their own narrow expectations, to scoop the lead roles in the school musical?

The premise of High School Musical is so simple it’s painful. But somehow this low-budget song and dance movie, aimed at tweenagers, those aged between 7 and 14, has become a cultural juggernaut. It arrives in the UK on September 22 on a slipstream of market-conquering statistics: it has been seen by 37 million viewers in the US, while the triple-platinum album is the year’s US No 1 album to date and top-selling album overall on iTunes and Amazon. More than 1.5 million single tracks have been sold digitally in the US, and last week HSM won two Emmies for Outstanding Children's Programme and Outstanding Choreography.

The prosperous “tweenager” has ensured HSM’s success — it is targeted squarely at the demographic and chimes with the current vogue for TV talent contests such as Pop Idol. Deliberately, it has something for everyone: highly sanitised Latino dance, bubblegum pop, rock, hip-hop, power ballads, soppy stuff, happy stuff.

HSM is conquering the globe. Disney is exporting it to more than 100 countries and it has already won record ratings in Asia and Australia. If Britain goes the same way, expect by Christmas playgrounds full of clean-cut Troys and Gabriellas, hymning the praises of multi-disciplinary learning and citizenship.

Its trick? HSM combines dashes of Romeo and Juliet, Grease and West Side Story with knowing, though ever-so-proper, nods to Mean Girls and Heathers. The peppy songs power the story, though I’d be amazed if, in 30 years’ time, today’s tweenagers will still be humming such HSM standards as What I’ve Been Looking For and We’re All In This Together in the same way that baby-boomers can recite a verse of You’re The One That I Want and Summer Nights.

It was shot, like most Disney TV movies, in barely any time with a cast that, while widely unknown, was familiar to the Disney audience. Its budget, $4.2 million (£2.2 million), was low. As with Cheetah Girls, another “multi-platform” success, Disney ensured that children could download free tunes from the film before its release, generating interest not only in the movie but also in the inevitable bestselling CD and DVD.

“Most ten-year-olds have computers in their bedrooms and that’s how they get their music these days,” says Rich Ross, the president of Disney Channel Worldwide. “The tweenage market was very small ten years ago but now the market is being driven by this age group.”

Children are already producing videos of themselves singing the songs and dancing to HSM on YouTube. One survey found that 61 per cent of adults defer to their “tweenage” children when buying music or DVDs.

“This is a considerable market for suppliers to tap into,” says Nicola Stuber, a researcher with Mintel, which published a report on the British tweenager in June. It found that 11-14s have a spending power of £1.7 billion and 7-10s just under £1 billion. About 10 per cent of 11-14s get £15 or more a week in pocket money. The tweenage market is said to be worth about £20 billion.

Where Grease was dirty under its perfect nails, HSM is ruthlessly buffed. Troy (Zac Efron) and Gabriella (Vanessa Anne Hudgens) share only the briefest peck. Indeed, their romance is displaced by a larger message: celebrate difference. Over and over again, Peter Barsocchini’s script extols the value of being secure in your identity, even if that identity is not simple or conventional. More than that: your peer group should value that difference. Everyone speaks in that “like totally dude” teen argot. HSM has moments of spikiness but is sugary enough to rot your teeth.

Ross says that it “whispers in your ear, not hammers into your head”, but that’s nonsense: one of the sassiest numbers, Stick to the Status Quo, which takes place in the school cafeteria, features an overweight brainiac revealing a love for hip-hop, while one of Troy’s basketball mates suddenly feels liberated enough to confess a love for baking. By the end of the film he has fulfilled his ambition of “making the perfect crème brûlée” and the structure of high-school cliques has been upended.

Ross says: “It taps into what kids are really thinking about — their place in the hierarchy, what friends think of them, where they fit in. Tweenagers are over-programmed now, especially by ambitious parents, either academically or with artistic or athletic pursuits. Troy’s dad has expectations of him, and for me the most moving scene of the film is when Troy tells him “I want to do what I want to do”.