A 15-year-old boy was hunted down and killed in front of commuters at a rail station in a "brutal and merciless" attack that was planned on Facebook, a court was told.
Sofyen Belamouadden was chased by up to 20 pupils from a rival school before he fell down the stairs to the ticket hall, where he lay helpless on the floor during the rush hour as he was stabbed, kicked and punched.
The Old Bailey in London was told that the heavily armed group planned the attack on March 25 last year to exact revenge for a "trivial slight" the previous day at Victoria station in central London, which they regarded as their 'turf'.
They are accused of using kitchen knives -- bought on the day of the killing for £3.99 at an Argos store by a 17-year-old boy and Victoria Osoteku (18), the only female to be charged.
One of the group was allegedly armed with a samurai sword while others carried batons and metal bars.
Abdeslam Belamouadden, Sofyen's father, left the court in tears as jurors were shown CCTV footage of him being set upon and left on the floor. The GCSE student was stabbed nine times, including in the lungs and chest.
Mark Heywood, prosecuting, said: "He was given no chance of life. So brazen and confident were his killers, we suggest, they openly carried the various weapons they used with them as they ran towards him and hunted him down. Such was their arrogance that they carried out that kind of attack in the heart of the capital city, in a public place, a terminus station at the height of the rush hour and in sight of scores, if not hundreds, of people.
"That confidence and arrogance came from knowing that between them they were so heavily armed that no other individual or smaller group, or even a police officer or member of station staff, could stop them. At the end of it all, they moved away, almost as one. He was so wounded by then, any recovery was beyond reach. His life simply ebbed away."
Twenty teenagers have been charged with murder, conspiracy to cause grievous bodily harm or violent disorder.
Mr Heywood told how the attack resulted from "simmering tensions" between the defendants, who went to different schools in London.
There had been a "short, almost inconsequential" fight the previous day, relating to an earlier feud over a girl.
That night, Mr Heywood said, there was a "gathering of troops and weapons" on the social networking site Facebook.