High School Musical 3 - Top of the class

FOR 14 hours they waited patiently in the rain, their pompons growing progressively soggier, their spirits undampened. Because, for the hundreds of screaming, wailing, crying teenage girls who lined up around Leicester Square in central London on Tuesday, their first glimpse in the flesh of a boy named Troy was, like, so totally worth it.

For those of us born before the first Gulf war, all this gnashing of teeth may seem somewhat perplexing. The object of this teen adoration, a young American actor named Zac Efron, 20, appears – with his highlighted hair and puppydog eyes – like any other wholesome-looking heart-throb. But all is not what it seems. For Efron, better known by his character name Troy, is the star of High School Musical, the most popular teen musical ever made.

High School Musical is truly a 21st-century phenomenon, a bubblegum-for-the-eyes extravaganza of all-American values. The first HSM was shown in January 2006 on the Disney Channel in the US, by a corporation unsure whether it had a hit on its hands. It did. The show – a made-for-TV feature-length movie – attracted 7.7 million viewers (very high by American standards), becoming the most successful film ever made by the channel and creating a spin-off soundtrack that turned into a bestselling album.

High School Musical 2, released a year later, broke even more records, with its premiere in August last year raking in 17.7 million viewers in the US. There have been a plethora of spin-offs, including computer games, reality shows, CD soundtracks and even a High School Musical On Ice tour. HSM 3: Senior Year, the cause of all the fuss this week, aims to take its popularity to a whole new level. For the first time, Disney – whose bosses must be thanking their lucky stars they took a chance on choreographer and director Kenny Ortega, who, since choreographing Dirty Dancing in 1987, has had precious few hits – is giving it a cinematic release with glossy, high production values and a budget of $13.3 million – more than three times that of the first one.

Of course, part of HSM's appeal is that it's so clean it squeaks. The plots, starring Efron as Troy and Vanessa Hudgens, 19, as his high school sweetheart, Gabriella, revolve around resisting peer pressure and rivalry, learning to work together and defeat the bad guys, all the while adopting a "singing and dancing solves everything, let's do the show right here" mentality. It is probably no accident that all three instalments were filmed in a high school in Salt Lake City, Utah, spiritual home of the Mormons and apple-pie American family values.

"High School Musical is sheer escapism for the kids who watch it," says agony aunt and author for teens Anita Naik. "They love the fantasy of it. Even eight-year-olds know it isn't real, school isn't like that, but they enjoy the music and the fun of it. For parents, it's one of the safer forms of escapism out there too – it's very wholesome, you can't really object to the values it promotes. It appeals to everyone."

HSM's knock-on effect has been palpable. For a start there's Britannia High (you see what they did there?), an ITV "musical drama" that will feature music composed by the likes of Take That's Gary Barlow and songwriting legend Guy Chambers. One of our TV previewers describes it as being "aimed squarely at 13-year-old girls and their fortysomething fathers", and it's populated by teens with names like Jez and Lola. Created by Strictly Come Dancing judge Arlene Phillips and theatre impresario David Ian, it's scheduled to start airing later this month.

And then there's Hairspray: The Musical (ah, they've done it there too, haven't they?), a reality show currently airing on Sky One that aims to pluck high-school children from their cosy nest of obscurity and propel them on to the West End stage in a production of Hairspray.

But hang on. Haven't we seen this all somewhere before? The school setting, the tendency to burst into song every time a heart is broken or an engine revved, the preppy, primary-coloured outfits? High School Musical has become the Grease of the 21st century, just as Fame was the Grease of the 1980s. And that, of course, is partly why it has become so popular. Let's not kid ourselves. High School Musical may be huge with teens and even preteens, but plenty of parents are peeking over their shoulders at the TV as well.

"Nostalgia is definitely part of the appeal of High School Musical," says Kate Harrison, the author of Old School Ties, a novel about tracking down old school friends. "We've all had that shared experience, even though, of course, most of us were zitty and tongue-tied, rather than witty and multitalented and utterly gorgeous, as the stars of these movies tend to be. It's also quite a good link between the generations: we can all appreciate the dread of detention, or remember how it felt to go on that first date, which means that, whether it's High School Musical or Clueless, we've got that in common."

Indeed, with so much of HSM's market being in DVDs and computer games that pre-teens are unlikely to have the money for themselves, it is undoubtedly true that it is Mum and Dad that have turned HSM from popular into phenomenal. But why the urge to revisit their school years? As anyone with half a memory knows, schooldays were often very far removed from the idealised image peddled by the likes of Sandy and Danny and Troy and Gabriella.

"When I was at school, I loved Grease exactly because it was nothing like school," says Anita Naik. "It was wonderful escapism, and I still feel like that about it now."

Harrison agrees. "We remember our schooldays as times free of stress and responsibility, when we had dreams – whether it was to sing and dance for a living, or travel the world, or just snog that cute boy in the year above. It's not necessarily true – I remember my exams as being pretty stressful – but our memories of that time are definitely rose-tinted."

So, with so many millions of devoted fans of all ages, is High School Musical 3 the end, or will there be an endless cycle of HSM movies – "High School Musical: The College Years", "High School Musical: Repaying the Student Loan", "High School Musical: The Pensioners"? Disney has so far kept schtum. But with such a highly successful money-spinner on their hands, the likelihood is that HSM 3 will be far from the final word from the world of High School Musical. It's enough to make you burst into song.

• High School Musical 3 is released in cinemas nationwide on 22 October.