The child soldier is a saddening image of Sudan's long and costly civil war.
Boys as young as 10 have fought on both sides in the conflict, which has taken up all but seven of the years since Africa's largest country gained independence from Britain in 1956.
The survivors have lost their education, as well as their innocence.
But thousands of young soldiers from both the mainly Muslim, northern-based government army, and the predominantly Christian, southern-based rebel forces, are returning to school in the capital Khartoum.
It is not uncommon to see a 23 year old, perhaps a veteran of 10 years of war, walking to one of the city's volunteer-run schools.
Classrooms are rudimentary
Like their younger counterparts, students wear white shirts and have different coloured trousers depending on their grade - brown for primary pupils and grey for seniors.
They have made their own way hundreds of miles from the battlegrounds of the south, many on foot.
Almost all work in the mornings to support themselves, attending lessons in the afternoons.
Earlier this year, Marco Wol Kwah, 18, left the southern forces - the Sudanese People's Liberation Army - and walked for a month and a half to Khartoum.
He is now a student at the Kinnetti Basic and Secondary School in the city centre.
Marco,who pays around $35 a year for his tuition, said: "I was in the army and I seemed to be going nowhere. The school is great. If I study, I can do something with my life later on.
"My favourite subject is geography and I want to be a pilot some day. Without an education, I can't do that.
"What I am studying here is useful to my ambitions."
The war has cost more than two million lives and crippled the economy to the extent that 92% of the population of 38 million live below the poverty line, while 30% are unemployed.
Louis Danga has gone on to become a doctor
Around four million Sudanese are described as "internally displaced", two million of them living in camps around Khartoum.
Most of Kinnetti's 311 students come from the south.
Dr Louis Edward Danga, a volunteer science teacher, was himself helped by the school and went on to become a GP.
He said: "Lots of the students have fought for the army on the side of the south, so many of them are 23 or older when they restart their learning.
"Almost all of the students had their schooling interrupted at some stage. For some, their schools closed down before they could finish, or maybe if they were in rebel-controlled areas affected by the war.
"Many come from the south to Khartoum. Students, or would-be students, are often left without options."
Considering the age of the pupils, the style of teaching at Kinnetti is very formal and traditional.
Sitting in rows, one group of mixed age was asked to name the continents and oceans of the world, which the teacher had numbered on the board.
The classrooms were tidy but the broken windows and makeshift chairs were a clear sign of under-funding.
The Sudanese government devotes just 3% of its expenditure to education. It is thought that 42% of children never receive any.
Of the remaining 58%, most do not finish basic school, the equivalent of UK primary level.
Dr Danga said: "Most of the students here are former drop-outs. This makes them very serious.
"Some have just discovered the importance of school. They say it's far better to be here. They come here and find they can get a better life with an education.
"Many here have done well. There are several who have gone on to university to become doctors or join other professions.
"Three years ago, we had a student here who was about 40, returning to education after many years away.
"At the same time, his son was here. He managed to finish before his father."
Dr Danga added: "Most of the students are forced to take responsibility for themselves, paying for their own lives. Often they have to work in the morning, so they can come to school in the evening.
"Unemployment is a big problem. Without qualifications you have a poor chance of finding a decent job.
"Also, there can be discrimination against people who have come from the south. Other than school, most would have no option but to stay in the army."
Acknowledging the need for students to earn money to support themselves, lessons at Kinnetti start at 2.15pm and end at 6pm.
The school caters for a wide range of pupils
Most students live on the periphery of Khartoum, commuting for up to six hours each day to save money on rent. In the worst cases, school fees are not charged.
A British charity, Education Action International, funds the teacher-training programme for Kinnetti, which was set up by local community groups in 1994.
It is named after a river in the south of Sudan, a symbol of most students' desire to return peacefully to their original homes.
The 1960s concrete buildings are rented from the government, which closed down another school on the site in 1993.
EAI's director, Cameron Bowles, said: "In a war, so much funding is diverted to the conflict that there is less available for education.
"The education system in Sudan has been decimated by the war. If you are born in Sudan, your likelihood of having even a basic education is very poor.
"When we turn on our television sets and look at the news headlines, we are seeing Iraq, Afghanistan and all the countries involved in the current war on terror."
He added: "Really what there should be is a war against poverty.
"There's often a feeling in Europe and America that people in the Africa are dependent on aid.
"When we come to Sudan and we see what's happening, we realise people are trying to educate themselves, so they can further their lives with as little help as possible."
Simon Akeen Akook, 19, epitomises this drive to gain an education.
He came to Kinnetti school after two years of service with the SPLA, walking for more than a month from the south to Khartoum.
He said: "The school is the key to life. I have three brothers in the southern army. They said 'Don't waste your time like this'.
"They told me to go to school. It's better than I had hoped. I was searching for an improvement, but this has exceeded all hopes.
"I would like to be a doctor and work in a hospital. The government isn't encouraging people to study like this."
Simon worries about the veterans who are not furthering their education.
"Their lives are bad. Many of them just smoke or drink liquor. They could complete their education if there were more schools."