By the BBC's Mark Doyle
Kauda, central Sudan
On a rocky mountaintop in central Sudan, schoolchildren are called to class by an old metal pipe suspended from an acacia tree. When hit with a stone it serves very well as the school bell.
I visited the school to find out more about life in the Nuba Mountains, a majestic granite range that straddles Sudan's north and south.
Most of the rebels' heavy weapons have been stored because of the ceasefire
The Nuba Mountains are a test case for Sudan. The peace deal which has been tentatively agreed for the region may be a template for other areas, particularly war-torn Darfur in the west.
Sudan is such a huge nation, the size of western Europe, that it is possible, confusingly, for there to be a devastating war taking place in Darfur and, at the same time, a peace process moving forward in other parts of the country.
A ceasefire was signed for the Nuba Mountains two years ago between the Khartoum government and the main rebel movement in southern Sudan, the Sudan Peoples' Liberation Army (SPLA). A few months ago, the next step in the southern peace process came; an agreement on power sharing in the Nuba Mountains.
The Nuba area is important because it is Sudan in microcosm. The people are mainly ethnic Africans, like most of the wider south, but also mainly Muslim, like most of the wider north.
If peace can be achieved in the Nuba Mountains, the theory goes, it might one day work in the whole of Sudan.
The ceasefire is holding. It is being successfully monitored by a mixture of international staff and Sudanese soldiers from the two sides who were until recently fighting each other.
This Joint Monitoring Commission, or JMC, is financed by a group of western countries known as "The Friends of the Nuba Mountains" and commanded by a Norwegian general.
The school I visited was in the town of Kauda, in territory controlled by the rebels of the SPLA. A few hours drive away - or a short hop in one of the helicopters of the Joint Monitoring Commission - is the government-controlled town of Kadugli.
The school here in Kauda is different from schools in areas controlled by the government in one crucial respect. Here, they teach in English. In government-held areas it is Arabic. Here, the SPLA has a governor; there, the government has a wali.
Ironically, given colonial history, the SPLA sees English as a language of liberation. The movement sees Arabic, coming from the Arab-dominated north, as a language of oppression.
Many Nubans, and others in the wider south, see Arabic as the vehicle which brought them Sharia law and an Islamist government in Khartoum, both of which they oppose.
The issue of language goes to the heart of the split between Sudan's north and south, a split which is often described as "Arabs versus Africans" but is in reality a heady mix of cultural, religious, ethnic and political differences.
The children at the mountain-top school, and the SPLA officials who showed me round, all said they only spoke English there.
Because of the war - which has raged here on and off for 50 years - there are hardly any local people with enough formal education to serve as English teachers, so the SPLA has imported teachers from neighbouring English-speaking countries like Kenya and Uganda.
They teach in rough stone classrooms with dirt floors and thatched roofs.
No Arabic is taught in this school
But conditions in this model school are better than in most of the rest of central and southern Sudan, where war has left many people living in absolute, total poverty.
Outside government-controlled garrison towns in the south there are hardly any roads, no factories and no large scale commercial agriculture.
"We only speak English here," said Juma Ibrahim, the SPLA schools inspector who accompanied me, "the children are not allowed to speak Arabic on school grounds".
This is a policy which mirrors another from an earlier age and a very different political dispensation.
When the whole of this area was controlled from Khartoum, Nuba children at secondary schools were discouraged from speaking their native African languages and encouraged by Arabic-speaking teachers from the north to only speak Arabic.
But English language as a policy is one thing; the reality on the ground is very different. As I was leaving the school, I heard some girls singing skipping songs in the playground. I asked Juma Ibrahim what the song was about.
Many ethnic Africans resent the "Arabisation" of their culture and language
"Its about a camel rider whose hat is blown off in the wind", he said "and the girls are singing 'Pick Up The Hat', 'Pick Up The Hat'."
I asked what language they were singing in.
"Well now, you see...," said Juma Ibrahim, with a smile and shrug, as if I had caught him out, "they are singing in Arabic".
The "Arabisation" of the Nuba Mountains, like other parts of central and southern Sudan, has been so profound that even people who say they are fighting for independence from Khartoum speak in Arabic. In the Nuba Mountains, SPLA soldiers bark orders at each other in Arabic.
While in Kauda, I received hospitality from a group of Sudanese aid workers running projects for the church charity Norwegian Church Aid.
The medical coordinator of the operation, Dr Richard Oleko, chaired the daily evening meeting of the aid workers in perfect English. But when he was chatting informally to friends, he used Arabic.
I spoke to one SPLA officer about the politics of language. He said that he sometimes felt bad that he was more fluent in Arabic than any other tongue - including his native African language.
"Sometimes", this officer said, "I feel ashamed of my own voice".
It was an extraordinary thing for a rough-hewn soldier to admit. But it gave a fleeting insight into one of the reasons why the Sudanese war has gone on, more or less continuously, for 50 years.