It was 5.14pm and the station was packed when the mayhem leading to Sofyen Belamouadden's murder began

It resembled an infantry charge, played out on the forecourt of a railway station and captured with brutal veracity by the hundreds of CCTV cameras put up to protect the public.

As the light faded and commuters began their journeys home on a March evening, two groups of teenagers were seen squaring up to each other outside Victoria station in central London, a court has heard.

Seconds later a young man slipped a hand into his jacket and flourished a samurai sword, which he held up before shouting: "We're going to fuck you up."

Following his call, the large group of teenagers at his rear pulled an armoury of weapons from their coats and bags – kitchen knives, flick knives, metal bars and batons – and together they charged, strung out in a line across the forecourt.

It was 5.14pm on 25 March 2010 and Sofyen Belamouadden, a 15-year-old schoolboy, had two minutes to live.

Outnumbered and outarmed, his group peeled off and ran for their lives, but Sofyen, a straggler on the edge of the fleeing teenagers, was grabbed from behind.

With a thud, he was rammed against the shutters of a cafe in Terminus Place, outside the station, before slipping away and sprinting towards the top of the escalator that led into the ticket hall.

By now seven or eight rivals were on his tail and another CCTV camera picked them up as they bundled and pushed their victim down the steps until he landed on his back at the foot of the escalator where he was set upon.

Within 12 seconds, Sofyen was dead. The moment – played on CCTV footage to five juries in open court – is a mass of arms and feet, body lunges and the menacing glimpse of weapons being thrust into his body.

Finally he is left alone and another figure approaches down the escalator, marked out by a large pink bag swinging from her side. Victoria Osoteku – the only girl in the group – stops to land a final blow with her foot to the body of the dying schoolboy, before taking off after her friends. A postmortem examination found 20 separate marks of injury on the body, including 11 incised or cutting wounds of which nine were individual stabs.

One wound to Sofyen's right shoulder was 12cm deep and had sliced into his right lung; another inflicted on the front of the chest had penetrated a rib, before wounding his heart and nicking one of his vertebrae.

Two stab wounds cut through his leg and there was a slash wound to the top of his head.

The onslaught – described by those who arranged it on social media the night before as "the madness" – was all the more shocking because until that moment the majority of the teenagers involved had never been in trouble with the police, the court has been told.

The court has heard that 18 of the 20 arrested and charged with the murder of Sofyen Belamouadden were A-level students at St Charles Catholic sixth-form college, west London. One was the son of a pastor and another a doctor's child, and their principal, Paul O'Shea, had expected many of them to go on to university.

The motive for the violence – like those found at the root of so many disputes which claim the lives of young people – was trivial, the court was told. It was not down to school rivalry, more a battle of territory in and around Victoria station, where groups of teenagers would gather in the evenings in fast-food outlets on their way home from their various schools.

The day before, there had been a spat between the same two groups of teenagers, one group from St Charles, the other from two or three west London schools, including Henry Compton, where Sofyen was studying for his GCSEs. In the dispute, one young man, Melvin Mensah, had been left with a bloody nose before the two groups were parted by the British transport police.

But the loss of face was sensed and the situation escalated in the hours afterwards when Mensah's friends from St Charles took to Facebook and their BlackBerry messaging system (BBM), to arm themselves and stoke an act of revenge which was to leave Sofyen dead. Pastor's son Femi Oderinwale, then 16, repeatedly asked that evening on Facebook for "a flick up ting".

He asked Osoteku – as the oldest in the group – to "buy some nanks from Argos", referring to a box set of kitchen knives. The 16-year-old later said the rivals would "get slumped". Other messages on BBM and via Facebook involved the group talking of the "madness" that would happen the following day. Another message read: "Everyone is tooled up."

Their conversations went on late into the evening of 24 March, and their talk was of exacting revenge and the need to "gather others to their case".

The following day they all were as good as their word, turning up at Victoria railway station by bus, armed with weapons, including the knives Osoteku had bought from Argos that lunchtime.

When their act of revenge was completed, the group, numbering around 20, left the area as they had come – on buses – some openly carrying their knives. Thirteen were arrested that evening on buses, with others being picked up later by police.

The planning of the attack on social media provided a key evidential chain for police who, the court was told, had examined mobile phones and thousands of hours of CCTV footage.

Twenty teenagers – including some who were not even within the station's ticket hall at the time of the killing – were eventually charged with murder.

Over five trials, which have taken place over the last 18 months, three teenagers, Samson Odegbune, now 19, Obi Nwokeh, now 20, and Christopher Omoregie, now 19, were convicted of murder and conspiracy to cause grievous bodily harm and given 18-year jail sentences.

Five others – Oderinwale, now 19, Samuel Roberts, now 19, Adonis Akra, now 19, Osoteku, now 20 and Bayode – were found guilty of manslaughter and conspiracy to cause grievous boldily harm. All, apart from Bayode, have been given 12-year jail terms.

Two others were convicted of conspiracy to cause grievous bodily harm and violent disorder, seven were convicted of violent disorder and three teenagers were acquitted of all charges.