Reshna Radiven, the VSO's head of volunteer recruitment, said demand for primary teachers with as little as a year's experience had doubled in the past three years. "There has been a surge in requests for primary teachers from all over the world," she said. "If enough volunteers do not apply we will not be able to meet the demand."
Figures released by the charity show that the age profile of teacher volunteers has changed. The proportion under the age of 30 fell by 23 per cent between 2000 and 2005, while the proportion aged over 55 increased by 14 per cent.
Roy Faulkner, 26, a primary teacher with a year-and-a-term's experience, helped to train teachers in the Gambia, West Africa.
"The majority of them are unqualified and paid a pittance," he said. "The classrooms tend to have been built by aid organisations some time ago and need repairs. If there is a blackboard in the room, there is often a lack of chalk. Textbooks date from the 1960s and have pages missing."
Mr Faulkner said he was given a month's training by the VSO in the capital, Banjul, before being sent inland to schools around the town of Janjanbureh.
Now back in north London, at Stroud Green primary in Finsbury Park, he said he had learnt a lot during his year.
"I have never had to teach with such limited resources. It helps you think on your feet and rely less on things like worksheets."
Despite living without electricity or running water, his allowance from the VSO meant he felt relatively wealthy. In the holidays he travelled to Senegal, Guinea and Mali, visiting Timbuktu.
Earlier this month, a guide produced jointly by the VSO and the National Governors' Association was sent to 23,000 chairs of primary and secondary school governing bodies. It aims to help deal with requests for sabbaticals to work with the VSO.
Judith Bennett, chair of the NGA, said: "Governors might think that if they have a recruitment and retention problem, they should not agree to sabbaticals. Experience has shown that sabbaticals can keep teachers at their schools for longer and encourage them to take on leadership roles."