Stripping back his trouser leg, Marco Luoh Kwan exposes a festering sore on the top of his foot. It is guinea-worm, a parasitic infection prevalent in poor communities without safe drinking water.
"It's hot as fire and painful all the time," he says. "The doctor tells me if I pay US$1 (59p) he will cure me."
But getting rid of the disease - which is rarely fatal but could result in Marco becoming disabled - is not his priority.
"All my money has been spent on school uniform and books," he says. The 18-year-old is a still a primary pupil at Kinnetti school, in the Sudanese capital Khartoum. He fled his hometown of Juba in the south of the country in 1997, after watching opposition soldiers kill his father during the country's civil war.
Now with no parental support, he says that his existence is an endless catalogue of expenses: schoolbooks and writing materials, as well as essentials such as food. His biggest outgoing is his $35 annual school fee.
"I don't have any savings," says his friend Simon Akeen Akook, 19, whose parents died in a famine when he was eight. After their death he walked across the country alone for 10 months in search of safety.
He survived on other people's cast-offs and by scavenging scraps of food.
Eventually he found shelter in a Khartoum refugee camp.
"I want to go to university and become a doctor. If we are all educated, these people who are against us now will work with us. Nothing is difficult with an education."
Civil war has raged in Sudan, Africa's largest country, since independence from Britain was secured in 1956. In 1983, the predominantly Arab government imposed Sharia law - the Islamic system of justice which forbids the consumption of alcohol or pork, and applies the death penalty for crimes against religious morality, such as adultery.
This sparked a long battle for independence by the Christian, Southern People's Liberation Army. Driven out by war, large numbers of southerners fled for the north. Almost half of Khartoum's population of five million are now displaced southerners.
School-age children are left to wander through the arid sandscape to find shelter in mud buildings in makeshift camps around Khartoum. At up to $200, fees at official government schools are prohibitively high for children who rarely have enough money for food.
While the Sudanese government has signed up to the international Education for All pledge to reduce illiteracy by 50 per cent by 2015, most of its income is ploughed directly back into the costly civil war.
Only about a quarter of children go to school - even fewer in conflict-ridden areas. In the South, just 15 per cent of the population is literate. So it has been left to local communities to ensure that their children receive an education.
Volunteers have been recruited to run basic schools, such as Kinnetti, in the refugee camps. Fees are minimal, and many teachers work with nothing more than a roof over their heads and a blackboard.
"A donkey cart brings around drinking water every day," says Elizabeth Venaiso, who heads Ffeda camp primary. "If it rains, the roads become too muddy for the donkey, and we have no water for the school. Our pupils suffer from malaria, diarrhoea and malnutrition. It makes it difficult for them to study."
The community organisations that run the schools are financed by a British charity, Education Action International. The aim is that, eventually, they should become self-sustaining.
"Communities are crying out for education," says Cameron Bowles, director of the charity. "They don't want to be dependent. They want to run their own lives. But they need the tools to do that. Education is a tool for the transformation of society."
Simon Akeen Akook knows this. He understands why his friend is willing to forfeit medical treatment for the sake of an education. He, too, chooses to forgo "luxuries" such as a change of clothing or a roof over his head.
"I have five years of school left," he says. "I can't go next year: I'll have to work to save up money for fees. It could take me 30 years to finish school. But I will not give up, education is the key to life."