From lessons in warfare to rehabilitation in classroom
But now the 17-year-old has exchanged gun and bullets for pen and notebook and enrolled as a pupil at a school in Freetown, the country's capital.
Memunatu Pratt, head of the conflict and peace studies department at the University of Sierra Leone, said: "He has problems controlling himself in school. Sometimes he can be violent towards his classmates. But you cannot rehabilitate people unless they are given an education."
Sierra Leone emerged from a brutal civil war in 2002 and offers an extreme example of the difficult circumstances Commonwealth educationists can face.
During the 11-year war, a large number of schoolchildren were, like John Kamara, press-ganged into joining the fighting forces.
Ms Pratt and her colleagues are now working to reintroduce these children back into the education system. This week she addressed delegates during a symposium on education in difficult circumstances. "Most children have lost parents," she said. "We have classes of 150 or 200 students, which can be difficult to manage and motivate. There's a lack of textbooks and high staff turnover."
Lack of facilities is a problem familiar to many Commonwealth countries.
Roselyn Wuniki, 28, told the conference that her three years working as an English teacher at Don Bosco secondary, in Papua New Guinea, have been a constant struggle for resources.
"My school is in one of the least-developed provinces in the country," she said. "There's a lack of teaching materials, so we have to run around looking for things to improvise with. Often this doesn't leave enough time to plan for lessons."
The lack of resources also has more dramatic effects. Despite taking on her job in January, Ms Wuniki did not receive her first pay cheque until the following December.
"I had to rely on my family. My older sister supported me," she said.
"There are times when you feel like giving up. But I chose to be a teacher and I love what I do, so I never miss a lesson."
Such situations are also familiar to Abdurrahman Umar, of the Nigerian national teachers' institute. He said: "In my country, a school might hire a secondary-school graduate, instead of a qualified teacher, purely to cut back costs. And we do not have enough teachers to implement universal education."
Memunatu Pratt acknowledges these are problems that few of the richer countries of the Commonwealth will ever have to face. But she believes there are valuable lessons for the developed world to learn.
"Education is about eliminating the prejudices and competition for resources that create conflict," she said.