The biology exam was only seven hours away and the crucial items for the practical were still missing. There was no other option: Philippa Hulme had to go and search for them herself.

Fortunately, the shrubs around her villa in Tanzania were home to several prize specimens and, before long, the science teacher had collected enough for the task in hand.

“We didn’t want the students to know which animal they were going to dissect for the exam because it would have given them an unfair advantage,” she says. “So we waited until they were asleep before hunting for the frogs.”

Thankfully, this was the first and only encounter of the croaking kind she had to deal with during a two-year teaching stint at Pugu secondary school near Dar-es-Salaam.

Teaching with Voluntary Service Overseas is never boring, as Hulme and colleagues Andrew Brown and Lynn Wadsworth discovered during their placements in Tanzania and Namibia.

Volunteering is rarely a choice for newly qualified teachers, but all three were fresh from the rigours of the PGCE year when they left Britain for southern Africa.

“It would not be appropriate for everyone but you learn an awful lot very quickly both from a personal and educational point of view,” says Andrew, who loved teaching in Namibia so much he stayed for four years. “You need to think carefully about the timing and your own reason for doing it.”

The charity may be best known for sending young adults to Africa to teach English, but that image is hopelessly out of date. Colonialist attitudes are long gone and VSO is now most interested in qualified and skilled recruits with “CIA” - creativity, innovation and adaptability.

Every year, about 80 teachers, more than half of them newly qualified, are sent out to teach alongside local education people in 60 countries across the world.

“We could place twice as many, so great is the need for trained teachers,” says Kate Purin, VSO’s education officer.

“Placements are usually for two years and, although experienced teachers are needed, those fresh out of college are still very welcome in shortage areas like maths and science.”

But teachers eager to take up a VSO place should appreciate that it is much more than an extended gap year - it is not an adventure nor a cheap holiday, says Ms Purin.

“The desire to escape the UK is not enough. Volunteers work alongside the local people, they live in the same conditions and eat the same food. Applicants must be sensitive to other cultures and be prepared to expect something new and different.”

Once through the application form stage, hopefuls attend an assessment day when VSO staff explain what they are letting themselves in for.

The applicants’ requirements are also considered - whether or not they want to work in an isolated region, for instance.

The process can take between two and nine months. Accommodation, flights, basic living expenses and National Insurance payments are covered by VSO.

But no amount of preparation can convey what it is like to teach in a developing country. Andrew, 30, who taught chemistry for a term after graduating, arrived at Nuuyoma senior secondary school in Namibia, near the Agolan border, with little idea of what he was going to experience.

Meanwhile, Lynn, 26, with only six months’ teaching experience, exchanged a resource-rich school for Onesi senior secondary with one textbook per class - and that was out of date.

Andrew headed a-level chemistry and physics students, while Lynn taught GCSE maths.

In Namibia and Tanzania, secondary schools are mainly boarding and only for the very able, but neither Onesi, Pugu nor Nuuyoma bear the faintest resemblance to traditional British boarding schools.

Volunteers were struck by the lack of equipment. Onesi was proud of its photocopier - although there was no paper to put in it. And A-level classes numbered 35 pupils.

But, says Lynn: “The Namibian pupils took all this in their stride. They were self-motivated and completely dedicated to learning.”

“A lot of students were extremely poor with poor levels of English,” says Andrew. “But I hadn’t anticipated running water and electricity, nor that everybody at the school would be so very enthusiastic about education.

“It was far from the image people have of lots of children learning under a tree with no resources.”

In addition, they all found that, with better discipline, fewer pastoral problems and considerably less red tape, there was much more time to concentrate on the real job of teaching.

“I didn’t expect my Tanzanian colleagues to be so friendly and helpful, nor the students so respectful and determined to succeed,” says Philippa, now head of year at John Bentley school in Wiltshire.

“Going to Tanzania had a profound effect on my teaching. It gave me the chance to develop my teaching style and build confidence in my subject without having to worry about keeping classes under control.”

A big surprise for all three was pupils’ attitude to asking questions. “Students were used to sitting in rigid rows and having a teacher lecture them while they took notes,” says Andrew. “The first time I asked them to do a practical, they looked in fear.”

All the teachers say their time in Africa was a major culture shock, but was far more satisfying than teaching back home. The volunteers found returning to Britain difficult after two years away.

“I had become part of the community,” says Philippa: “In Tanzania, the important things were a new birth or a wedding, whether the water was working or if the corn was growing.

“Arriving back home where people were chattering about a new CD or what they’d seen on television was very difficult to take in.”

Advice for would-be volunteers? Do plenty of reading on the country that you want to go to and, if possible, talk to someone who has been there.

Accept that the school you end up in will have nothing like the resources you take for granted in the UK. Volunteering is not for those who cannot adapt to change or who are addicted to routine.

“You get to live with people you wouldn’t normally share a flat with, but even that can be a learning and interesting experience,” says Andrew.

As for pay and conditions, would-be volunteers shouldn’t worry: “I was more than able to live on the pound;200 a month I got,” says Andrew. “I managed to save enough to travel to all parts of southern Africa during the school holidays.”

VSO, 317 Putney Bridge Road, London SW15. Tel: 020 8780 7500 Web: www.vso.org.uk