Chennai - or Madras as it was once called - is India’s fourth-largest city and home to 4.2 million people. A thriving metropolis that specialises in car building and information technology, it hosts European companies like BMW, which unveiled its first assembly plant there last year. But head outside for three hours and any hint of urban sophistication disappears, the cityscape gives way to vast tracts of countryside, arid plains and lush padi fields.

Here, often hidden and tenuously connected to the rest of civilisation by churned-up dirt tracks, are villages of mud huts where people live a different kind of life, usually with no running water, no sanitation and, at best, intermittent electricity. Their access to health care, education and employment is limited, and most are illiterate. They scrape a living as bonded labourers in agriculture, rice mills, brick kiln industries and sugarcane fields.

This is the lot of the adivasi, or the tribal people of India - 500 communities. Traditionally forest-based, they have been forced out. The state of Tamil Nadu, on the east coast of southern India, is home to 0.65 million of them, around 1 per cent of its population. Tourists are usually oblivious to their existence, but this summer 12 pupils from Monifieth High in Angus met the Irula tribe.

“It was very important to us that the group added value,” explains Kelly McAulay, head of international operations with Signpost International, who led the trip.

There can be a tendency for school trips to be polished packages, admits Jeremy Morris, a depute headteacher at Monifieth High who accompanied pupils on the trip, along with principal teacher of science Rhona Goss. “You go and look but don’t get involved,” he explains.

In India, the pupils got their hands dirty - literally. But their presence alone made an impact that no one had predicted.

The adivasi are the “lowest of the low” in Indian society. “They are considered to be highly polluted and for higher-caste neighbours it’s like having vermin living on their doorsteps,” Ms McAulay explains.

As a result, they are shy and have low expectations. To Ms Goss, they were the “invisible people”: “They are outwith Indian society and nobody cares.”

Receiving western visitors gave them a much-needed sense of importance. “The confidence they got from the simple fact that white people had bothered to visit was amazing and surpassed all our expectations,” says Ms McAulay. “In some of the villages, the pupils were the first white people they had seen.”

Everywhere they went, pupils and teachers had an entourage. “It was like being a celebrity,” adds Ms Goss.

But they were celebrities who were ready to muck in. The pupils did some construction work, helping repair “concrete monstrosities” that interspersed the mud huts. During breaks they supped coconut milk instead of tea, thanks to an agile lad who shimmied up a tree.

Mostly, they worked at night schools run in the village streets by Signpost International in conjunction with an Indian charity, the People’s Education and Rights Trust, teaching English to children. Signpost and Pert hope that by improving literacy, they will enable the adivasi to become fully-fledged members of Indian society, able to access their rights - such as the right to an education and the right to vote.

But the night schools are just one part of the project, which is funded by the Lloyds TSB Foundation, and run in 20 villages. “Our work with them is about explaining their rights and empowering them. Increasing literacy is part of that,” explains Fiona Dixon, PR manager for Signpost International.

The Monifieth pupils were the teachers but it was always Signpost International’s intention that they should learn something. They wanted the young people to see the value of teaching others how to shape and build their own future as opposed to giving hand-outs which, Ms McAulay says, can do more harm than good. They also wanted to create “global citizens” who would make responsible decisions about their own lifestyles. “We wanted to return with 12 change-makers.”

The pupils worked in groups of four, preparing lessons in the afternoon, then delivering them in the village streets from 5pm. Each group worked in a different village.

Sixteen-year-olds Michelle Kelly, Calum Dwyer, Jenny Tonner and Lorna Grubb had been expecting to teach around 15 children aged four to 14. They had been warned that because they were taking the classes, “there might be a few more”. In the end, 64 children turned up.

The teenagers coped until a rabbit was thrown into the mix. “A man just turned up on a motorbike and put this rabbit in among the children,” Michelle remembers. “Obviously they were distracted!”

But the young teachers improvised. “We were doing colours, so we started describing the rabbit and asking them what colour it was and what kind of animal it was,” she adds.

They also held lessons about animals, body parts, family and adjectives. They were equipped with flashcards and other resources, and paid for blackboards to be bought before they arrived. All the educational materials were left behind and the Monifieth pupils - each of whom raised Pounds 1,800 - supplied school bags, pens and paper. Even their teaching methods caught on.

“It’s all rote learning but the way our kids taught them tended to be active learning and participation,” says Mr Morris. “They saw the value of doing it that way.”

The money that paid for the group’s food and accommodation will also have a lasting effect. New pots and pans were bought and an additional floor - two rooms and a kitchenette - was added to Pert’s headquarters, so the pupils had a place to stay. This level was commandeered by the girls who slept on inflatable mats, while the offices served as sleeping quarters for the boys and Mr Morris.

But the gift that made an instant impact was a bore well, for which the school paid, providing an entire community with access to water.

“One woman described the years of humiliation and oppression whenever she went to get water in a neighbouring village,” he says. “She said to us, ‘We will remember your faces until the day we die’.”

The money for the well became available when Calum Deuchar called a bus company and asked for transport to Glasgow Airport for free. “The money for a bus journey to Glasgow - 70 miles or whatever - has provided them with water for life,” he says, struck by the impact his call has had on an entire community.

During their three weeks in India, no one got homesick - they were too busy. The humidity was hard to cope with, though, admits Calum, especially during long bus journeys on bumpy roads. Eating curry and rice for breakfast, lunch and dinner led to cravings for, in Michelle’s case, “a plain cheese sandwich”. Every so often, however, the western world made contact via mobile phones. Text messages and calls brought up-dates on major events back home, including the Big Brother evictions and the death of Hollyoaks’s Max.

But the trip has made the pupils look differently on life at home and changed their perceptions. Chris Holder, 17, feels that the way the media reports on life in the developing world skews reality. Far from people being “sad and miserable”, he felt they demonstrated “a stronger side to humanity”.

Calum was surprised to find that people with so little were still so happy - not at all what he had been led to believe, he said; the West could learn from them. “In the western world we are all about work and money. But over there, they have a proper community; everyone speaks to each other because they are not in their concrete houses with their TVs on, not speaking to anyone.”

It is not the strange stories of rabbits invading English lessons that will stick with her, Michelle says, but the people. Calum agrees: “These are people you will never meet again but who you will continue to meet for the rest of your life, if you know what I mean?”