Phil Hearne was head of the London Academy in Edgware, north-west London, when Hannad Hasan, 17, stabbed Kiyan Prince, 15, with a penknife outside the school gates last year.
Hasan claimed the killing was an accident, but he was convicted of murder last week. Fifteen teenagers have been killed in London in the past six months.
In his first interview since Hasan's conviction, Mr Hearne said: "The sense of guilt, the feeling that you could have done something, has stayed with me and the staff who manned the school gates at the time and rushed to help him.
"Even if there's nothing you could have done, the guilt stays, simply because they were in your care at the time. They were your responsibility."
The experience of leading a school after the death of the Queen's Park Rangers youth team player last year had been a big test of his leadership, he said.
"The day after the death, there was silence," said the 47-year-old former history teacher. "There were a thousand people in shock, with pupils and teachers crying. This sort of extreme situation tests your mettle. People look to you as an individual and I wonder if we as heads are fully prepared for that."
After the conviction Mr Hearne called for a national figure to provide leadership on the issue of youth violence, as Jamie Oliver had taken the lead on poor school food.
He told The TES this week: "We still have a lot of small projects going on a local scale, but we need some concerted movement forward which is more than piecemeal.
"The Metropolitan Police's knife amnesty started after Kiyan's death, but several months later we had a series of stabbings again. I don't have the answers, but kneejerk responses are not good enough."
Mr Hearne is supportive of new laws allowing teachers to search pupils for weapons and believes security guards provide valuable reassurance for pupils. "We introduced a police officer to the school five or six years ago and I think it has been good for the school and the community," he said.
The London Academy owned two metal-detector wands but never used them. Intelligence picked up by teachers on the ground was often enough to help stifle any potential violence. "You have to have senior staff who are prepared to put their ears to the ground," Mr Hearne said.
His plea comes a week after Sir Ian Blair, the Metropolitan Police commissioner, announced a new blitz on gang violence in all 32 London boroughs.
Mr Hearne, who oversaw Edgware school's transformation into the London Academy in 2004, is about to become head of the Paddington Academy in west London.
SCHOOLBOY VICTIMS OF KNIFE ATTACKS
Kiyan Prince (above) was stabbed to death by an older pupil in Edgware, north west London, in May last year.
Ben Hitchcock, 16, died after being stabbed during a fight in Beckenham, south London, in June. He had just sat his GCSE exams.
A 14-year-old boy was stabbed in the shoulder as he walked to Acton high in west London, in June. His injuries were not life-threatening. Two 12-year-olds were arrested.
Kodjo Yenga, 16, an A-level student at St Charles sixth form college in North Kensington, west London, was stabbed to death in March. Eight teenagers have been charged.
Luke Walmsley, 14, was stabbed to death during a break in lessons at Birkbeck school in North Somercotes, Lincolnshire, in 2003.