Gilly Turner is halfway up a craggy rock face, her left knee doubled and somewhere near her left ear. Her right arm is outstretched, fingers digging into a crack, while her left hand explores the hard, vertical surface for a hold. Her cheek is squashed against the grey rock, and there’s an intense look in her bright, searching eyes.

Geography teacher Sarah Stone stands on the sun-dashed ground some 6m below. “You can do it, Gilly,” she calls to the 17-year-old. Above them, beyond an overhanging ledge and outgrowing trees, the sharp blue Alberta sky is as big as a prairie. “Use your legs, Gilly. Stay close to the rock.”

The disembodied voice of climbing instructor Brett Lawrence ripples down the rope attached to Gilly’s harness from a lip of rock about 9m above.

With an almighty effort she hauls herself up one more step, her fingers finding another slim slit to grip, her toes curling into another inch-wide indentation.

It’s 11am on a clear summer’s day, and Gilly is clinging to the side of Heart Creek valley, a tributary of the Kananaskis in the foothills of the Rocky Mountains. It’s a couple of hours’ drive west of Calgary, in Canada’s Alberta province. A Wiltshire schoolgirl, Gilly is one of 12 British teenagers selected by the Army Cadet Force (ACF) from all over the UK to join 150 Canadian cadets for six weeks’ wilderness training.

“I love it here,” says Gilly, back on firm ground. “This place is fantastic. I’ve done a bit of climbing before, but never anything like this, and never anywhere like this.”

Gilly, a boarder at Godolphin school in Salisbury, joined her school-based cadet force three years ago. Like the other 11 Britons here, she was selected for this trip ahead of 80 other cadets recommended by their “detachment” leaders. “I’ve been on two-week camps in Britain before, but this is the best summer I’ve had,” she says “I’m doing stuff I love for a whole six weeks, and it’s free.”

Chris Wooton, also 17, is a pupil at Dollar Academy, Clackmannanshire.

“We’re so lucky; you just can’t get anything like this at home,” he says, gesturing at the magnificent granite-topped mountains around him. “To get this level of adventure training you’d have to go on a special course, and it would never be in such a unique setting. This place is stunning, fantastic.”

The course is based at the purpose-built Rocky Mountain army cadet training centre, one of three camps in Canada attended by British cadets each summer. The trip costs them a nominal pound;200 each, and those lucky enough to be chosen get expert instruction in adventure activities such as hiking, rock climbing, canoeing, kayaking, mountain biking and glacier walking. They also learn leadership skills and take part in military-style “PT” and “drill” exercises. It’s part of an exchange programme between the British ACF and the Royal Canadian army cadets that’s been going since the 1960s.

Major Rod Boswell oversees the British end of the exchange programme and selects the cadets. “The primary aim of the cadet force is to give as many kids as possible the chance to do something they wouldn’t normally do,” he says. “These are the ‘best of the best’ of 70,000 cadets in the UK. They are already high achievers in the force, and usually at school and sports as well. The aim of this trip is to give them new experiences, teach them new skills, and help them develop into adults.”

Education aside, it sounds the perfect outdoor holiday, an ideal way to de-stress after taking AS or A-levels and fill the gap before starting upper-sixth or college. What could be more away from it all than hiking to 9,000 feet, looking across miles of lakes, rivers and mountain ranges; camping out in bear country; traversing glaciers; or pedalling down woodland tracks and through mountain streams without seeing a car for miles?

It’s not a bad way to escape for a teacher either. Ms Stone, a teacher at Cheltenham Ladies College, Gloucestershire, is one of two adult cadet leaders accompanying the 12 young people on their summer adventure. Ms Stone - or Captain Stone, to use her army moniker - became a cadet leader three years ago when she started the first all-female cadet unit in Britain, at Godolphin school, where she was teaching at the time.

About 250 independent schools in Britain have their own Combined Cadet Force (CCF) units. Although she had no military background, Ms Stone was pleased to be offered some responsibility. “It was a steep learning curve,” she says. “But I like being around kids - and the enthusiasm you get from seeing them progress is fantastic.”

Nevertheless, after a hard year’s teaching, the decision to spend six weeks of the summer with a group of hyperactive teens wasn’t an easy one. “I did have to think about it,” she says. “But in the end it was an opportunity I couldn’t refuse.”

The lure of the spectacular Rockies and the chance to try a host of activities she’d never get the time for at home was too good to resist. “I wasn’t sure what to expect but it’s beaten all expectations. People pay thousands of pounds to do this sort of thing, so it’s a real privilege.”

Her fee of pound;50 a day doesn’t go amiss either, although Ms Stone says the trip was an educational one, as much for her as for the young people.

“They learn a lot about themselves,” she says. “These are very condensed courses, so they are taking on a massive amount in a short time - that means it’s a mental challenge too. I’ve learned things that I’d never have had the chance to otherwise. I’ve gone through the same process as them, of self-doubt and gaining self-confidence.

“Also, it’s a good thing for a teacher to have another way of interacting with kids. When you come and do something like this, you don’t get stale, which you can if you’re stuck in the classroom. Doing something in your holidays revives your energy levels. It’s made me a more dynamic teacher.”

Ms Stone is one of 7,000 adult volunteers who run the 1,700 community-based ACF and 250 school-based CCF detachments throughout the UK. Although funded by the Army and the Ministry of Defence, the cadets is not merely a recruitment wing for the forces, although it has definite military habits, such as uniforms, ranks, marching drills, weapons training, and an overuse of “TLAs” (three-letter acronyms).

Founded by social reformer Octavia Hill more than 130 years ago to keep London street children out of trouble, it sees itself as a “grass roots” youth movement with “a strong claim to be the largest teenage organisation in the country”, according to PR officer Steve Thomas. With 40,000 ACF members and another 30,000 in the CCF, that may well be true, although Mr Thomas is aware that it can suffer from a somewhat old-fashioned image. “It used to be seen as a cross between Dad’s Army and the Hitler youth,” he says. “But we are a modern, dynamic youth organisation. It’s not about kids with short back and sides, in battle dress and guns, but about young people learning skills for life.”

While most of the young people on the Rockies trip are public-school educated and university-bound, that’s not true of all of them.

Tom Hull and Luca Miceli are resting on Rosen Ridge, almost 2,500m above sea level, gazing back at the glassy face of Rosen Lake 610m below, from where they’ve just led a group of their Canadian colleagues. Tom, a sixth-former at Huntington comprehensive school in York, says the cadets have given him a sense of achievement he never gained from school. “I chose to prioritise the cadets over school because I enjoyed it more and was better at it,” he says.

Tom gave up history A-level because he couldn’t fit the coursework in around ACF weekends away doing training courses. He became the first master cadet in his county for three years and was chosen for the prestigious Rockies trip, which, he says, will be more useful for his intended outdoor studies college course in the Lake District than the extra A-level.

“This course has made me a more patient person too,” he says, referring to the language barrier between English speakers and the many Francophone Canadian teenagers. “It’s taught me about listening and communication.”

For Luca, an A-level student at Suffolk college, for whom university seems like “too much education”, the cadets was initially attractive because of “the military thing, guns, and all that”. “But,” he says, looking out on the incredible landscape, “you can also say you’ve done something worthwhile with your life. And this is a once-in-a-lifetime opportunity.”

For more information about the Army Cadet Force, tel: 0845 600 7799; www.armycadets.com