Pattaya, widely known as the sex capital of the world, is an unlikely destination for a school visit. But that’s where four Year 10 girls from Banbury’s Blessed George Napier school in Oxfordshire spent a week earlier this year.

Pattaya is a down at heel resort with a not particularly impressive beach.

It’s in Thailand, which is 12 long - and expensive - flying hours away. In short, it’s not a place that would top anyone’s list of ideal school trip destinations.

There is no “red light” district in Pattaya, there are simply places where the sex trade is more obvious than others. There are bars, massage parlours, brothels and prostitutes. On the sea front, there’s a clandestine trade in child sex. One estimate suggests that this medium-sized seaside town has more than 30,000 sex workers.

On Pattaya’s streets there is an eclectic mix of foreigners - known locally as farangs. These are not the young backpackers who can be seen elsewhere in Thailand. The typical farang is a solitary European male in his forties or fifties, often with a pretty Thai teenager in tow.

Prostitution is big business, and the Russian mafia is heavily involved in controlling the sex trade here. No visitor to Pattaya could fail to see it, and the Pattaya Mail must be one of the few local newspapers in the world to run cartoons about street prostitution.

Inevitably, there are child casualties - abandoned, at risk. The orphanage has acquired a worldwide reputation as a sanctuary, a place walled off from the unsavoury realities beyond its gates. This is what the George Napier girls had come to see.

Their visit started as a competition entry that involved writing a proposal for a development project, explains Sue Moss, George Napier’s assistant head and the teacher who accompanied the girls on the trip. “We were studying development in geography and as a result we made contact with the UK fundraising operation for the Pattaya orphanage. We undertook to raise funds to convert a building into a street children’s centre.”

The George Napier students didn’t win the competition, but their proposal attracted a lot of interest, resulting in an invitation to visit the orphanage, with free flights courtesy of British Airways.

The girls knew about Pattaya’s reputation, but they weren’t prepared for the reality. “I had no idea that it would be so in your face,” said Camilla Concannon, 15.

Across the street, a European in his 50s is arm in arm with a Thai teenager ,who looks about Camilla’s age. “At first I didn’t realise what we were seeing, that there are so many homeless kids,” she said.

The girls stayed in the orphanage compound. They were never in any danger, partly because of the high walls and security guards and partly because the crime syndicates who run Pattaya ensure that the streets are almost completely free of conventional crime.

The orphanage isn’t a Western-style children’s home operating from a single building. It’s a huge undertaking running on four large sites. It was originally set up by Father Ray Brennan, an American Roman Catholic priest who was sent to Thailand during the Vietnam war.

In the 1960s, US navy ships would call in at Thai ports as part of their R and R - rest and recreation - from service in Vietnam. That kick-started the sex trade and also pulled in thousands of rural families to work in the bars and tourist traps.

Father Ray was sent to Pattaya as a temporary replacement for the parish priest. Soon there was a small bundle on the doorstep, an abandoned baby.

But instead of following the previous practice and sending the child to a state orphanage, Father Ray decided to take in the child. More followed and the orphanage was born.

That was 1973. Other projects followed, starting with an old people’s home, schools for deaf and blind children, a vocational school for young disabled people, a job placement agency for the school’s graduates, and a home for street children. Father Ray died last year, but 750 children and young people are now in the care of the orphanage. Thousands more have passed through.

The George Napier girls were to work on a new day centre for street children in town. The centre aims to offer a daytime sanctuary for children who are unlikely to cope with the disciplined, ordered world of the orphanage. The four English girls planned to create a garden and paint a wall mural during their stay. They worked in 90C heat alongside children who led very different lives.

“It just showed us how much we take for granted,” said Becky Cook, 14. “We have made a difference. Seeing the kids at the centre today was really good. Kids enjoying themselves instead of being trapped on the streets.”

Becky and her fellow travellers are keen to return to Pattaya as volunteers. But it wasn’t all about work. The orphanage organised trips, one to a more salubrious beach, others to a dolphinarium and an elephant park. At the end of the week, the English girls were guests of honour when the street centre was formally opened with a Thai ceremony.

“I did have some concerns about what we would find when we got here,” says Sue Moss. “But the visit has been a success. The girls have got on well, shown a good deal of maturity and experienced things they may never experience again. Isn’t that what education is all about?”

The Pattaya orphanage relies entirely on voluntary work and donations: www.pattayaorphanage.org.ukwww.britishairways.com