The boy, who gave himself the pseudonym Jan, was one of 148 Kurdish schoolboys who were held for five months by Isis extremists in Syria and subjected to an indoctrination programme designed to brainwash them.
The boys were in a convoy of 250 students - mostly from the Kurdish border town of Kobane which is now under Isis siege - stopped in late May as they were being ferried back from taking middle school exams in Aleppo.
They were repeatedly beaten as well as indoctrinated, according to accounts given to the Daily Telegraph and a Human Rights Watch report released on Wednesday.
But the jihadists' tactics may also have worked, judging by the account Jan gave after being taken by his family to safety in Turkey.
When Isis fighters stopped the convoy, the girls were released within a few hours, but the boys were taken to nearby Manbij.
"At first they took us to a mosque and told us they just wanted to check if we were good Muslims, and that we would be released after one day," he said. "But the next day we were taken to a school."
Aged between 13 and 15, the boys were kept 15 to a locked room, and the jihadists assigned one hostage in each cell the role of "emir" or leader.
"We prayed five times per day, including the 4am morning prayer, read the Koran, had religious lessons and special classes in Islamic State ideology," said Jan.
The plan was enforced using a mix of kindness and brutality. The boys' teacher was a Jordanian named Surukhan al-Tabuki, a man in his 30s who tried to befriend the students and become their confidant: "He was kind to us, unless someone made a mistake in his lessons or behaved badly and then he was ordered to send us for punishment."
The "punishments" were conducted by other men, jihadists with no names who wore black face masks.
When Jan used the Kurdish word for God instead of the Arabic "Allah" he was tortured.
"They strung me up by my arms, hanging me by a rope that was tied to my wrists. They practised karate and kick-boxing on me," he said.
Another boy, who asked to be called Ivan, said he was beaten when he accidentally dropped a notebook in which he had been writing Koranic verses close to a waste bin.
The violence was random, Jan said. "Once the jihadists told me, 'We want to kill you. How would you like to die? Here in front of your friends or alone?'"
Shortly after they were kidnapped, a group of boys managed to escape, climbing up a ladder on to the school roof and hopping over the wall.
Ivan said this was his most frightening moment: "The boys escaped from my cell. I stayed behind because I didn't want to leave my cousin, who was in another classroom. I was punished for that."
After that the kidnappers tried to turn the boys against one another, "recruiting" some of them as "spies", the boys said.
Sometimes they were allowed to play football, but at others they were made to watch videos showing massacres of "kaffirs" - unbelievers - by Isis jihadists.
The children were slowly released in groups, often with little explanation. Jan and Ivan were among the last group of 25 freed last week.
Both said most of the captives had also come to believe in Isis' ideology, and five had even chosen to stay and join the fighting.
"Sometimes I am confused," said Jan. "But everything they said they proved using passages from the Koran.
"The Koran tells us that Muslims will rule the world. Isis are right: there will be a global Islamic caliphate."
