Complete silence. This was the response when I told my 10-year-old daughter Phoebe that we were off to Bromley to see High School Musical, the stage show. Her mute reaction was not because she was ignorant of it; quite the reverse. For Phoebe and her peers, HSM has achieved so profound a cultural importance that to be taken to the actual event in the flesh is a treat of such eminence that mere words are pointless.

She and her friends know every song; they sing them while queueing for lunch, which they will eat out of their HSM sandwich boxes, wearing their HSM sweatshirts.

At the Churchill theatre, Bromley, where the £30 tickets sold out within days and where the show’s national tour has begun before a 10-week residence in the West End (with a £9m advance), the foyer was crammed with chirping girls brandishing an excess of HSM merchandise ferociously marketed by Disney representatives. The fan base they have hit is something of a bull’s-eye: that huge group of tween-age girls who have pocket money to spend and wardrobes to fill.

Alice Tappenden, 13, cheerfully admitted she loves HSM so ardently that she wishes she was a pupil at East High in Albuquerque, HSM’s fictional setting. “It’s so cool. They get to wear their own clothes and have lockers,” she sighed.

Tracey, her mother, agreed. “School certainly seems more enjoyable in America. They seem to have more going on in terms of after-school activities.”

Indeed, while the plot is a classic boy-meets-girl romance, HSM is a wildly enthusiastic salute to the institution of the American high school. All the iconography is there: the Stars and Stripes in the gym, cheerleaders and the dining hall filled with classic characters straight out of the year book: studious Brainiacs, athletic Jocks and luvvie Thespians, all of whom run rings around the faculty (that’s the teaching staff), singing and dancing as they go.

Parental influence is invisible, everyone speaks in slang and no one gets mugged. It’s a safe, enclosed world where you grab a Coke, study maths and fall in love, probably for the first time and certainly with someone of the same racial background.

We have been here before. Indeed, just about the only things that differentiate HSM from Grease are that no one smokes and everyone has a mobile phone. Yet unusually for Disney, HSM was a hit that came from nowhere. It started life as a TV film, first shown on the Disney Channel in January 2006. It was watched by an estimated 7.7m people and 12 hours later scored 500,000 hits on the web-site; 1.2m copies of the HSM DVD were sold in the first six days of release. The soundtrack went quadruple platinum, all this with little overt publicity and no known stars.

How do you describe a cultural phenomenon, I asked Arnaud Cazet and Laurent Bentata, from Stage Entertainment, co-producer of the stage show with Disney. Bentata and Cazet were over from Paris for a sneak preview of the British theatrical release before a possible French version. “You can measure it by TV ratings, DVD and CD sales,” said Cazet. “But a phenomenon is about more than that: it’s about word of mouth, whether children are talking about it at school and singing the songs in the playground.”

Perhaps it’s the show’s overt chastity that appeals: the main protagonists, Troy and Gabriella, seem to achieve their kicks largely by holding hands. “There is nothing corrupt or smutty about it,” agreed Sarah Grieves, who had brought her 11-year-old daughter Emily along.

“It all looks so lovely and the sun is always shining,” said Keeley Leamon, whose 13-year-old daughter Ellie admitted to having seen the original movie and its sequel at least 20 times. “It’s all very nice, when you consider the other stuff that children like.”

For all its feel-good stardust, HSM does have a core of credibility. American high schools work hard to create their own small universe, an arena of instantly recognisable iconography in which everyone knows where they are. HSM, with its battles between the Brainiacs, the Jocks and the Thespians, acknowledges and celebrates this. While outwardly urging its characters to react against the status quo, it quietly reinforces the importance of fitting in by virtue of its own unrevolutionary zeal.

The appetite for shows like HSM that reveal high school as a sanctuary of unchanging standards is perhaps because of the notorious instability of American family life. Disasters such as the Columbine shootings reinforce the fondness America has for its high schools and what they represent. I write with some authority, as for my gap year I was a pupil at an American high school whose raison d’être was straight out of the pages of HSM. Indeed, I spent much of my year at Cushing Academy, a boarding school in Massachu-setts, half believing I had stumbled onto a film set.

There were lockers, cheerleaders and innocent pranks involving the faculty. We had the same slang, wore the same clothes and did all the same things as in High School Musical; indeed, we even put on a musical. All right, it was by Rogers and Hart and therefore had slightly better tunes, but our production of Camelot shared many an HSM moment, even down to the main actress being ousted by a newcomer (er, yours truly) and the overlying “everyone shall be involved” ethos.

All shall be involved, everyone has a talent, follow your dreams, turn up to rehearsals on time and life will turn out well. This is the underlying message of HSM. It was the underlying message of my real-life American high school and I can see why some British teenagers fervently wish that their secondary school was East High, Albuquerque.

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