It was a decisive moment. The smartly dressed black man stood in the bus door blocking our departure. “I believe in Jesus.” He waved his clipboard. “Do you believe in Jesus? Are you Christians?” He stared, angry. “I must live too. ” This was not an invitation to join the brethren but the final, humiliating, shot in a cheap hustle. He was met with stony-faced silence.

Thanks to fuddled minds and dysfunctional Spanish we missed our guide at Caracas airport, and this plausible English speaker wanted to help. He also wanted paying for his unofficial services. But when he fixed us a bus at $9 a head - 800 per cent more than the going rate - the 12 exhausted students stared into the smiling eyes of chaos, and faced it down. They hired their own.

The doors closed and the hulk wheezed off up the mountainside, past the giant cigarette hoardings, past the “ranchos” or shanty towns hunched perilously above the dual carriageway, on to Caracas itself and the start of a month-long exploration.

From 500 metres up on the steep green slopes of the El Avila mountains, the city is almost appealing. Caracas is best seen this way. At street-child level it is a sprawl of decaying Americana. Rusting gas guzzlers, Dodges and Chevrolets, bounce through the pot holes, a reminder that fuel was once as cheap as clean water.

When we finally caught up with our Caracas guide, Cecilia Salinas, she told us that she paid 500 Bolivares (70p) for a full tank last year. Now the same tank-load approaches a fiver. Once the most stable country in Latin America, Venezuela has seen recession, unemployment, riots and even an attempted coup in recent times.

But the students, eight boys and four girls from the New Forest in Hampshire, had not spent 18 months planning for this. It was with relief that, conspicuously laden, they squeezed through the milling crowds of the city’s bus station, said by the guide books to be one of the most dangerous places in Venezuela. The bus was cramped, leaky, mechanically suspect and driven by someone with no regard for the laws of probability. But it was welcome as the means of getting to Merida, a city 1,000 metres up in the Andes, and base camp for the high altitude trek. Venezuela has not seen the point of building railways.

World Challenge Expeditions organised trips for some 60 schools this past summer, fixing destinations from the popular Inca trail in Peru to the isolated forests of Borneo. A further 80 are preparing to set off next year. Most of them are from the state sector, despite the popular preconception that exotic travel is only for the rich kids of private education. In this case it was the students of Totton and Brockenhurst Colleges near Southampton who decided to spend the best part of two years each raising Pounds 2,200 so they could blow it on the holiday of a lifetime.

In theory they controlled really important things, the itinerary and the spending, while World Challenge took charge of the mechanics. The company supplied essential kit like tents and the lightweight Tranja cookers. It provided training sessions, a qualified team leader, air tickets and a budget. Perhaps most importantly, World Challenge set up comprehensive medical and rescue insurance. The team was even burdened with something like a huge walkie talkie with a flashing light on top - a “satellite link” which we were told would send helicopters zooming straight for us in the event of disaster. Or horseplay. This made the trip expensive by anybody’s standards: but it secured the parents’ blessing. Without it, the kids would have been allowed no further than Ibiza.

Moreover they had a whale of a time, clocking up a list of extraordinary sights and experiences. There was the startling, table-topped Mt Roraima rising 2,000 metres out of the grasslands of south-east Venezuela. This is one of the oldest and strangest things in South America, a last fragment of plateau from when America and Africa were one land mass. There was also the vastness of the Orinoco river, the improbable gulch made up entirely of the semi-precious green jasper where the team could slide and swim; and the giant eagle which swooped to within touching distance as they stood, stared and ducked.

By the time their month was up they had covered huge tracts of the country, from the Andes next to Colombia in the west, to Roraima and the other flat-topped mountains near the border with Brazil and Guyana, and on to the Caribbean coast to relax. There was also drama: the collapse of an exhausted student, followed by urgent (and successful) medical attention. And the coach crash that had seemed inevitable with so many mad drivers about.

Where Caracas was disturbingly different, Merida seemed familiar: a small, neo-colonial city of red pantile roofs and narrow streets in a grid. The altitude and the mountains took the edge off the heat and Merida felt almost European, except perhaps for the ghostly webs of Spanish moss which festoon even the telephone wires. It is a popular tourist resort, but even here there is the smell of Venezuela’s troubling slide: two years ago the city had the highest funicular in the world. That was until a cable snapped, sending a car and two passengers plunging to oblivion.

It still wasn’t fixed when, dizzy with altitude sickness, we tumbled into the penultimate station, 4,000 metres up, in the Andes. After three days’ hiking we had reached the “Pass of the Cross” in the Sierra Nevada - the gloomy, rock-filled high point (4,250 metres) in a four-day trek up the mountains overlooking Merida.

The students had been expecting something Amazonian, but this was far too high for the steaming humidity of rain forest. The attractions were of a subtler kind: a series of rapidly changing micro-climates and varying topographies amid, of course, the breathtaking immensity of the mountains all around.

We went from valleys where the Caribbean winds have swept the orange shale of all but the sparsest plant life, to slopes of profusion where giant ferns arch over paths shiny with scattered mica, and our guide, Volkhard George, could slip into the undergrowth to point out the camouflaged orchids. There was no rain forest, but plenty of cloud forest, its dense green foliage contrasting starkly with the white formations drifting by and the bright blue sky. There is a permanent dripping in these woods, half way up the mountains, where the paths are muddy and blocked with creepers, but where for us the cool made walking easy.

The first two days of the trek had taken us up to 2,600 metres and Los Nevados - literally “the snow capped peaks”, although the amount of snow actually visible from the small farming village has declined as the glaciers recede.

Tourists are not such an unusual sight here, although the children came to stare. Los Nevados is a standard stopover on the hikes run by Merida travel agents. Not that visitors have made the village rich. Families sleep in one room and the bare-earth kitchens boast little more than a cooker. The village still farms maize, potatoes and the occasional cow, using the mules for ploughing, and for carrying the packs of European travellers.

Ours included. The farmers put us to shame as the 10-mule train pounded up the mountain track. They wore yellow Wellington boots, stuffed with newspapers for socks. The mules set off behind us and, with two packs per beast plus the weight of our weakest walker, they reached the summit some hours ahead. We meanwhile were staggering up the last 300 metres of cloud-shrouded scree, fuzzy with lack of energy and oxygen at a height roughly four times that of Ben Nevis.

It was worth it just to wake up at the Pena’s farm the next day. The sky had cleared after torrential rain, and the Andes were in plain view stretching away, vast and snow capped into the sunlight. When he was alive, Grandfather Pena had been the first man to climb the 5,000m Pico Bolivar, the highest in Venezuela (although Colombia has a Pico Bolivar even higher), and one of those who helped carry, in sections, a statue of the liberator to the top.

These days the family makes its living from people like us who camp on their land, and from seasonal work on the cable car. Like the people of Los Nevados they are in a curious way part of what could be described as the Venezuelan heritage trade. The only sign of modernity in their extraordinary kitchen was a single light bulb and a battery-powered radio hanging on a nail. Everything else ran off the wood fire that filled the room with a faint blue haze and left the walls as black as the floor of beaten earth trodden to a shine. This kitchen was where the worst victims of altitude sickness were gathered together for his “tea”, a tasty but sickly infusion of herbs gathered on the way, and a great amount of sugar. Next door was a single room where three generations slept together.

The missing element in this Totton and Brockenhurst expedition was the project. This is meant to be a central feature of most World Challenge trips, the projects are frequently concerned with basic health or education. For example, groups visiting Peru this year have worked on renovating a church and on building a classroom in a shanty town.

Venezuela also has its established projects. The Jesuit’s Fe Y Alegria charitable school at Merida receives regular World Challenge visits and one of our group members will spend the next twelve months teaching there. With this exception, however, the New Forest team rather by-passed the projects business. They had been planning to count parrots on behalf of something called the World Parrot Trust; but this somehow did not quite come off.

This was apparently untypical; but then so was the team, being for the most part older than the average, and more determined on adventure. Two were already at university and the rest had just finished A-levels. Generally World Challenge works with groups of 16 and 17-year-olds.

Some of our group had never flown before, but none had problems adjusting to the strange conditions. In fact they could probably have coped with a more relaxed regime than the one imposed by World Challenge. The self-consciously democratic team structure meant that different posts, “assistant supplies” for example, were rotated on a daily basis.

We had a new leader every morning, which may have been a good thing. And a newly baffled accountant, which was certainly a bad thing. Every evening we sat in a circle outside the tents for an organised “debrief”. Fortunately, we defied the temptation to talk aimlessly at length.

There were only a few problems. The lack of good Spanish or a properly organised project probably made it hard to meet Venezuelans, which was a shame. More troublesome was the wide difference in fitness between the majority, and one or two others. Most of the team, the boys in particular, were serious sportsmen andor experienced walkers. Some had taken part in the 50-mile Ten Tors slog across Dartmoor organised by the army. Four or five were keen to join the forces. But there were others who found the walking tough, particularly when loaded up with the mountainous rucksacks (20 kilos-plus). Correctly, they stayed at the head of each trek. But walking at someone else’s halting pace, with constant stops and starts, is exhausting and very probably a factor in the altitude sickness.

Many on the team thought that better training in England would have helped; no one had anticipated how heavy the packs would feel, or the heat of the sun. But a disparity in fitness levels is a common problem on these trips. The fact is that, at Pounds 2,200 a shot, the group members are self-selecting. It is difficult to exclude anyone prepared to make 18 months of solid commitment and willing to raise the cash through countless evenings’ working at Tesco’s.

Working predominantly with schools, World Challenge is keen to emphasise the educational benefits of its trips. Like the rather more rugged Operation Raleigh - for real adventure nuts - it has quite a military influence: the founder and many of the staff come from a services background. And, also like Raleigh, it believes not only that travel broadens the mind but that large doses of physical exertion mixed with team work make for a better brand of youth.

They may be right. Certainly the participants from this World Challenge trip came back happy and confident, with a sense of independence - and a finer understanding of the workings of the human bowel.

Nicholas Pyke was a guest of World Challenge Expeditions. The standard school-based expedition is for 16 to 18-year-olds, but the company also runs: “First Challenge” (shorter trips for students aged 14-16); “Ultimate Challenge” (allowing individuals from different parts of the country to get together); and “Gap Challenge” (offering three to six-month work placements for older students abroad). Tel: 0181 961 1122