The neon lights of a city street at night look harsh on the CCTV footage.
A boy senses trouble and runs, but he is quickly caught by a group of young men and goes down under a flurry of blows.
As the CCTV camera picture sharpens, we see one of the attackers is holding a knife - not just any knife but a machete.
He rains blows down on his helpless victim before casually jogging away and disposing of the weapon in a bin.
Those images seem a world away from a cramped classroom where 12 kids sit at a round table working industriously.
This is the Youth Education Support Service in Southwark, which provides teaching for young people who have fallen out of mainstream education or those who want to boost their current studies.
The children told us they don't use knives, but in their world, violence and the threat of violence is an ever present danger.
David is a 15 year old boy who wants to be a lawyer. He has a medium build, is neatly dressed and is a little shy. He doesn't speak with any great emotion.
He doesn't try and impress in the telling of his stories. He speaks with the bored resignation of a teenager who has been given detention. And he tells me that all of his friends have been held up at knife point.
His classmate Laura left school because she was threatened there with a knife. Chantelle tended the wound of a boy who was stabbed at her school.
Charlotte has a picture of her shiny knife on her shiny mobile phone. How could something so unimaginable to me, be so entirely normal for them?
There are very few figures on the scale of knife use. The British Crime Survey has an entire chapter on gun crime. But there is no comparative analysis for knife crime.
And yet we do know that knives and other sharp weapons kill at least three times the number of people that guns do.
The lack of firm figures comes as no surprise to Tony Bleetman, an A&E consultant at Heartlands hospital in Birmingham. He sees many more slash and stab wounds each year than gunshot injuries.
Knives can be concealed in belt buckles
"We often find that stab victims are very reluctant to tell the Police or for us to tell the Police. Not quite sure why that happens perhaps because they want to sort their own problems out or keep that within their group or perhaps its for fear of retribution."
Research shows that just a third of knife attacks treated in hospitals are reported to police. As he flicks through the sequence of pictures he saves for his lectures to medical students, I wince and turn away. This is as bad as anything I saw in Iraq.
"We must remember that gun crime in this country is still relatively quite rare", he remarks. "It doesn't happen as nearly as often as we see these awful knife attacks.
But for whatever reason gun crime seems to grab the headlines and this sort of stuff tends not to."
Knife attacks on children by other children did make front page news last year when a 14 year old boy called Luke Walmsley was stabbed to death in school by another pupil.
The image of Luke blowing bubbles was in every paper - the picture of an adolescent making his way from childhood to manhood.
His parents Paul and Jayne Walmsley imagined, as most parents would that a small secondary school in the Lincolnshire countryside would mean their son - their pride and joy - would be safe.
But there are few parts of the country which have been untouched by this phenomenon. Our researchers spoke to teenagers stabbed by other teenagers in Plymouth, Exmouth, South Wales, and Manchester and Newcastle.
There are piecemeal efforts by some to try and tackle the phenomenon. Kevin Everard runs a group called "Be Safe" which is funded by local authorities and schools to come in and teach young people about what knives can do.
It isn't a touchy feely "knives are bad" course for good kids. They work in areas where carrying a knife is commonplace. In five years, 4000 young people have been on the course. The adolescents they deal with are streetwise. Many of them will carry a knife.
At one of the sessions five sets of teenage eyes are narrow and wary. Their conversation is forceful and insistent. But they are visibly shocked at the gruesome pictures Kevin displays.
They show fresh knife wounds. The responses from his charges become more hushed, less certain.
"Self-defence is still the main reason for carrying", says Kevin. "They live in areas where they feel they could be vulnerable to 'jackings' as they call it, which is mugging or robbery".
He is able to sift through a bizarre armoury. There are flick knives and butterfly knives which are expensive looking. But there are concealed knives also; in belt buckles, combs, a lipstick.
Slashed across face
They have been carefully crafted to cause the maximum damage, and I wonder what drives someone to carry these, never mind create them.
"Something that is worrying me more and more is the small value that they sometimes put on their own lives," he said.
"Now if they're putting a very small value on their own life, then someone else's life may be worth even cheaper, and that can obviously lead to terrible repercussions."
Those repercussions are forever etched into the pale Scottish face of Kirsty Nesbit. She is 16, and wide eyed, and instantly likeable. A week after her 15th birthday she was out with a group of friends.
She fell out with one of them, and they had a tussle. Her "friend" slashed her across her face with a craft knife.
Kirsty needed 93 stitches. The scars stretch from the top of her forehead to her lower lip, and right across the fleshy part of her left cheek.
The pictures of Kirsty just after the attack take my breath away. Those bloody marks are so incongruous on the face of a child so young. I wasn't the only one.
Kirsty's face was used in Edinburgh by the police to try and shock people into not carrying weapons. It certainly made an impact on the city.
Superintendent Stephen Grieve of Lothian and Borders police believes that people need to know the damage that knives can do - especially in the hands of children.
"What's changed in the issue is the emergence of young people, 11, 12 and 13 years old, carrying and being willing to use offensive weapons.
That's something as a policeman that I've noticed certainly over the past five to 10 years."
Panorama: Your child's been stabbed was broadcast at 22:15 BST on Sunday, October 17 on BBC One.