An American school that raised pound;30,000 to buy the freedom of child slaves in Sudan has unwittingly triggered an international row over the practice.
A class at Highline community school in Aurora, Colorado, was studying the history of slavery in America when they read a newspaper story about the existence of Sudanese slavery.
Kyle Vincent, a pupil, explained: "When we read the article, it just hit us. I just got the feeling, I've got to stop this." Her fellow students started collecting money to free one or two slaves. Soon, cheques began pouring in.
But when their efforts were broadcast on CBS television, the United Nations Children's Fund (UNICEF) issued a public statement decrying the buy-back practice of anti-slavery groups as "intolerable".
There is growing concern at the resumption of slavery in war-torn Sudan, but many commentators fear that such buy-backs merely fuel the practice.
The history of slavery is one of the major causes of the long-running civil war in a country divided between black African southerners, who were the main victims, and Arab northerners who went south searching for slaves. Britain ended the trade after its 1898 conquest of Sudan, but it has returned in a conflict fought chiefly between rival tribal militias.
The Khartoum government asked UNICEF to investigate following recent rescue missions by Christian Solidarity International in Bahr el Ghazal - scene of last year's devastating famine. CSI bought the freedom of about 5,000 slaves at pound;30 a head.
"Paying for freedom is not intolerable," said Charles Jacobs of the American Anti Slavery Group. "What is intolerable is to leave these women and children in the hands of brutal captors."
Carol Bellamy, director of UNICEF worldwide, said: "While we understand the humanitarian instincts of schoolchildren to purchase the freedom of slaves, the sobering truth is that these efforts will not end the enslavement of human beings. It does not address the underlying causes of slavery in Sudan: the on-going civil war and its by-products of criminality."
Jemera Rone of Human Rights Watch explained the dangers: "When the practice started in the mid-1980s, the primary motivation of the raiders was to acquire cattle, with slavery as a secondary consideration. The availability of foreign funds poses the risk that those who conduct the raids may abduct children and women for the purpose of gain from the sale."