The man named as extremist "Jihadi John" received anger management therapy in his first year of secondary school after getting into fights, a former teacher has said.
Mohammed Emwazi, who has been seen in beheading videos of Western hostages, attended Quintin Kynaston school in St John's Wood, London, a decade ago.
The teacher said he needed help controlling his emotions.
But he was regarded as a "success story" after the therapy, she added.
Emwazi, a Kuwaiti-born British man, who is in his mid-20s and from west London, was identified as the person dubbed "Jihadi John" earlier this week.
Security services have been criticised for being aware of Emwazi but not preventing him from joining Islamic State.
Speaking to BBC Two's Newsnight, the teacher, who was not identified, said the former pupil had been a "lovely, lovely boy" who had a "real willingness to try and succeed".
"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," the teacher 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.
She said he did not come from a troubled background, left school with qualifications and had "every chance of doing well".
"I just can't believe he'd do that," she said, adding that he had achieved everything he wanted to do at school.
"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."
The teacher also said MI5 had interviewed his former teachers from the school.
A statement from Quintin Kynaston school said it was "shocked and sickened" that its former pupil may be involved with Islamic State.
It added: "All members of staff at QK work very hard to support the education and well-being of our students and protect them from harm.
"In this respect, QK has been extremely proactive in working with the government's Prevent strategy for a period of time and will continue to be so for the foreseeable future."
'Apology for terror'
UK-based advocacy group Cage has suggested that MI5 may have contributed to the radicalisation of Emwazi.
Downing Street said that suggestion was "completely reprehensible", while London mayor Boris Johnson described Cage's comments as "an apology for terror".
Mr Cameron has defended the UK's security services, praising the work of "these extraordinary men and women".
He went on to say the security services' "dedication and work has saved us from plots on the streets of the UK that could have done us immense damage" within the last few months.
Mohammed Emwazi's movements before heading to Syria
- 1. Aug 2009, refused entry to Tanzania: travels to Tanzania with two friends, but is refused entry at Dar es Salaam. Tanzanian police have denied Emwazi's name is on their database of suspected foreign criminals detained and deported in 2009, as he had claimed. Emwazi and his friends are put on flight to Amsterdam, where they are questioned. They return to Dover and are questioned again.
- 2. Sept 2009, travels to Kuwait for work: leaves the UK for Kuwait for work.
- 3. May/June 2010, returns to UK for holiday: he returns to the UK for an eight-day visit.
- 4. July 2010, refused re-entry to Kuwait: Emwazi returns to the UK once more for a couple of days. He is stopped at Heathrow on his return to Kuwait and told he cannot travel as his visa has expired.
- 5. 2013, travels to Syria: Emwazi changes his name to Mohammed al-Ayan and attempts to travel to Kuwait but is stopped and questioned. Three days later, he heads abroad. Police later inform his family he has travelled to Syria.