"Is it a smash? Is it a sensation?” Steve Fickinger, VP of theatrical licensing at Disney ponders. “No. There's only one word left for High School Musical: it's a phenomenon.”

In a few days, the live stage version of Disney's made-for-TV musical arrives at the Hammersmith Apollo in London. Meanwhile, the sell-out national tour, which kicked off in January, continues unabated, with a visually identical but in fact slightly different cast. And according to Jeff Calhoun, the show's director, this summer's High School Musical mania may be only a hint of things to come. “Is it the new Grease?” he asks. “It might very well turn out to be.”

At this point, it's difficult to remember that there may be some people who, despite being alive and possessed of a television, have no idea what I'm talking about. And that, before a trip to the Birmingham Hippodrome to witness the mass, preteen HSM hysteria in ear-piercing action, I too had never heard of East High.

For all eight of you left, these are the particulars: on September 22, 2006, Disney aired a modestly budgeted (£2 million) musical about life in an Albuquerque High School on the UK Disney Channel. Relocated to the cleanest, most racially harmonious, least offensive educational establishment in history is the greatest story already told: Romeo and Juliet (Grease, West Side Story). The insuperably divided clans in question are brain v brawn. Troy is a basketball dude. Gabriella is a “maths genius freak girl”. At a new year's karaoke party, they discover an embarrassing passion for singing pop ballads together in harmony and in public.

But back at East High, would they, could they, pursue this passion to its logical conclusion and... audition for their high-school musical? Seemingly insurmountable barriers bedevil their union. He's way too cool, she's way too shy;they're both, oddly, way too busy. And the evil drama club president Sharpay and her teen queen twin Ryan are determined to deny this alpha male, prom queen pairing its shot at school stardom.

If you think you know what happens next, you probably do, but the effect of this coming-of-(st)age tale registered on the Richter scale on the tweenage population of every country with a Disney Channel. It began breaking records from the minute it aired, and shows no signs of stopping. The film has now been seen by 255 million people in more than 100 countries in the world. Israel, Australia and the Netherlands are all currently casting for their own touring productions, to open in the autumn, and France, Italy and Germany will soon follow suit. A stage version of the sequel, High School Musical 2 (same characters, same crises, slightly different setting: a golf club), will premiere in November in Atlanta, Georgia.

In the Apollo run, the part of Ms Darbus, drama teacher, devotee of the great god Thespis, detester of the mobile phone and all-round, loveable nut, will be played by EastEnders' own Letitia Dean. Taking a break from rehearsals, 41-year-old Dean confesses to being “terrified” at the prospect of the opening.

“Next to this” she says, “EastEnders is a drop in the ocean. Friends were hyperventilating when I told them I'd got the part. I'm now hyperventilating at the thought of how much singing and high-energy dancing I've got to do, with a cast of lithe youths, in front of 3,500 people. Yes. It's fair to say I'm s****ing myself.”

But as to why High School Musical is so bewilderingly popular, Dean can't entirely say. “It's hit some sort of crazy nerve, but I'm not sure which one.”

It's not, apparently, the nerve of Disney's corporate prep marketing, the cunning, “dedicated micro sites” on the web, the mind-boggling, 670 affiliated HSM products, on ice shows, reality TV spin-offs, or Nintendo games. Because, as Fickinger puts it: “We're not shy about promoting our shows at Disney. If we could repeat High School's success on everything we make, you can be very sure we would.”

Which can only mean that what makes High School Musical so preternaturally popular are its fundamentals. In which case, Calhoun could be right. High School Musical could be here for the cultural long haul.

Kenny Ortega directed both TV films, and is currently filming the third testament: High School Musical: Senior Year, which is due for nationwide cinematic release in October, and stars Zac Efron and Vanessa Anne Hudgens, the original Troy and Gabriella.

Ortega was the choreographer on Dirty Dancing and, when asked about the key to his more recent venture's success, replied, “What's true of Dirty Dancing is true of High School Musical. You have to be real. You have to be repeatable.” He made very sure that all of Baby's moves in Dirty Dancing could credibly be pulled off in the bedrooms of real teenagers everywhere. The same is true of High School Musical; there's even a “how to” special feature on the DVD, which walks through one of Ryan and Sharpay's numbers.

Then there's the music. The insanely catchy, cleverly multigenred soundtrack, by six teams of composers, has now gone quadruple platinum, despite none of its singles ever having featured on a radio Top 40 countdown.

But perhaps the key to HSM's unstoppable success is to be found in the screenplay, by Peter Barsocchini, which deviates from inherited protocol in one important particular. It isn't Grease. It isn't Grease not only because nobody smokes, nobody skips class, and teen pregnancy is as far removed from the realm of possibility in East High as Danny Zuko pitching up to school in a people carrier. More fundamentally, while Troy starts out a stud and Gabriella is a bit square, they're both allowed to stay that way. Gabriella's love of maths and Troy's addiction to basketball doesn't stop them from loving each other, and something as entirely unexpected as musical theatre.

HSM's central message, then, is that pigeonholing people is a very bad idea. And that, it seems, is the tweenage nerve of note that Disney has been hitting so consistently since 2006.

“I now know,” Fickinger concludes, “that the magic of Disney characters is that they're who kids want to be. They want to be Troy and Gabriella and get up in front of the school and chase their dreams. They have begun to take their place in the pantheon of great Disney characters, and that's why the stage versions work so well, and why they work without Zac Efron and Vanessa Anne Hudgens in the leads. Cinderella exists beyond whoever plays her. And so, now, do Troy and Gabriella.”

The stage adaptation, by David Simpatico, is not a scene-for-scene replica of the film, but it's close enough that you might not notice at a squint. The major scriptural deviation comes in the form of a school radio station, which allows for a narrator to chivvy proceedings along. The largest loss is the basketball sequence, which is hinted at, rather than replicated on stage. Gone too is a hilarious Silence of the Lambs spoof, and an inspired gag about Michael Crawford and what Troy can hope to achieve in life if he adheres to his inexplicable conversion to the totally lame-ass world of musical theatre.

The stage show, then, takes itself a touch more seriously than its filmic predecessor, and runs at half an hour longer. But what fans need to know has nothing to do with increased character development arcs, expanded emotional nuance, or the heightened political edge to the school arts/sports debate.

Let me then cut to the chase by saying that yes, every song from the original is very much present and correct, in virtually the same order, along with two new ones. That the plot zips along with eye- watering speed, the dancing is, if anything, tighter, and that, despite not being the sainted Efron and Hudgens, the stars of the touring show more than filled their strangely dowdy costumes.

Twinkly eyed 23-year-old Mark Evans, who is to play Troy in the Apollo show, thinks the appeal of the show lies in its crowd energy: “I've done a fair bit of musical theatre, but the speed of this show is just incredible,” he says. “The buzz of this young cast, with a live band, blasting this music out to 3,500 screaming kids, I think the atmosphere's going to be amazing, I really do.”

On the night I caught the show, fans were as effervescently enthusiastic as Disney could have dreamed. Ellie, a nine-year-old from Wolverhampton in full cheerleader regalia, said that this was the second time she'd been to a theatre. The first was the previous week, when she'd come with her mum to see High School Musical. Is it as good as the film? “Better,” she says. Why? “My DVD's broken.”

Her younger sister Aleisha, 7, also sporting pom poms, expands: “High School Musical is our most favourite thing; we liked this a lot.”

Dexter, despite having no trouble at all with his home technology, agrees. “It's brilliantly done” he says, “I've seen the film over 30 times. Is it odd to hear a 31-year-old straight man admit to that?” A little, Dexter, yes. “Oh well. It's just fantastic live. Sharpay is completely classic.”

Is the story enough of a classic, though, to earn an enduring place in the ranks of Disney's great, era-defining stage adaptations, alongside Mary Poppins and The Lion King? Or will the seemingly inexhaustible thirst for the franchise ebb once the final film's been canned, the second musical arrives, and the kids move on to new, uncharted obsessions?

“Right now,” according to Calhoun, “it's impossible to say. Only time will tell if this is something future generations will relate to. But I know that when the curtain goes up, people are screaming for the film. When it goes down, they're screaming for the show. When that happens, you know you've really got something. All we can do is ride this wave and see where it takes us.”

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