A former teacher of Mohammed Emwazi has said the Islamic State (Isis) militant received anger management therapy in his first year of secondary school, the BBC reported on Saturday.
Until his identity was revealed on Thursday, Emwazi was known as "Jihadi John" due to a series of videos in which he was seen beheading Western hostages and issuing demands to foreign governments.
The unidentified teacher at the Quintin Kynaston school in Queens Park, London, told BBC's Newsnight that her former pupil had counselling in his first year of secondary school after getting into fights.
"We'd find that he'd get very angry and worked up and it would take him a long time to calm himself down, so we did a lot of work as a school to help him with his anger and to control his emotions," she said. "It seemed to work. He had a lot of respect for all of the work that had been done for him at our school."
The teacher described the young Emwazi as a "lovely, lovely boy" who had a "real willingness to try and succeed".
"I just can't believe he'd do that," she added. "He went to a university of his choice, and from the way he started in year seven to how he blossomed until he left at the end of sixth form was a huge achievement for him."
A statement from the school said it was "shocked and sickened" that Emwazi may be involved with Islamic State.
"There has been much press speculation in recent days concerning allegations about a former student Mohammed Emwazi, who left almost nine years ago. If the allegations are true we are all extremely shocked and sickened by the news."
It added: "All members of staff at QK work very hard to support the education and wellbeing of our students and protect them from harm."
British security services have reportedly known Jihadi John's identity for some time, but it was suppressed for "operational reasons".
Prime Minister David Cameron defended security services after they were criticised for failing to prevent Emwazi from joining Isis.
"They are having to make incredibly difficult judgements, and I think basically they make very good judgements on our behalf," Cameron said.
"I think while we are in the middle of this vast effort to make sure British citizens are safe, the most important thing is to get behind them."
Mohammed Emwazi has a ruthless reputation within Islamic State, where he is referred to as Abu Abdullah al-Britani.
"I saw him many times," one former member told The Guardian. "I knew it was him from his build and profile. The people with me knew who he was too. But we could not ask questions.
"That would have been too dangerous. To us, he was the British guy who slaughters people."