Sofyen Belamouadden was hunted down by an armed group of 20 teenagers before being stabbed nine times during rush hour at Victoria station, jurors at the Old Bailey court were told.
Witnesses saw youths with weapons including a samurai sword with a blade 20-30 cm long, a flick knife and a Swiss Army knife, and machetes and screwdrivers.
Sofyen was killed after tensions arose between pupils from two London schools.
The attack was in revenge for a skirmish at Victoria the previous day when a boy from a rival school had been left with a bloody nose, the Daily Mail reported.
During Facebook chats that evening, some of Sofyen's alleged killers discussed what was to happen the following day, and how to get weapons, the jurors were told.
Prosecutor Mark Heywood said the attack took place in broad daylight in front of hundreds of commuters. On the day of the attack in March last year, two of the accused left school at lunchtime to buy a block of knives.
Samuel Roberts from Camberwell, Obi Nwokeh from Bermondsey, Enoch Amoah from Camberwell, and Victoria Osoteku from Deptford, all aged 18, have each denied charges of murder.
They also pleaded not guilty to conspiracy to cause grievous bodily harm.
Details of the trial were published for the first time on Wednesday after reporting restrictions were lifted.
Sofyen was "given no chance of life", said Heywood. "So brazen and confident were his killers that they openly carried the various weapons that they used with them as they ran towards him and together 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 passing by, people going to their own homes." he said.
"They were so heavily armed that no other individual or smaller group or even police officer or member of station staff could withstand them or stop them," he added.
The victim and his friends were "hopelessly outnumbered".
"They had already lost the arms race and it was obvious that they had seriously underestimated what they were likely to meet at Victoria that afternoon. They did not expect the kind of weapons and they almost certainly did not expect the level of ferocity," the prosecutor said.