The only female member of a gang that hunted down and killed a schoolboy in front of hundreds of commuters at Victoria station in London has been jailed for 12 years.

Victoria Osoteku had helped set up the confrontation on Facebook the night before 15-year-old Sofyen Belamouadden's death in an attack which was captured by CCTV cameras in the station.

Osoteku, 18 at the time of the killing, is the 13th teenager to be sentenced in what is one of the most high-profile teenage gang killings of recent times. Her jail term brings the total sentences handed down to those responsible to 124 years.

Osoteku was convicted of manslaughter after a four-month trial in which she gave evidence for 21 days ? the longest testimony at the Old Bailey in recent years. She was also found guilty of conspiracy to commit grievous bodily harm.

Sentencing her on Thursday to 12 years in a young offender institution, Judge Christopher Moss QC told her: "You played a pivotal role in the events of and leading up to that day and must take a substantial share of the responsibility for that."

The attack was the result of "simmering tensions" between students at a sixth-form college in Ladbroke Grove, west London, and Sofyen's fellow pupils at Henry Compton school in Fulham, which exploded in the violent attack on 25 March 2010. The day before, the two groups had confronted each other at the station and one youth was left with a bloody nose.

Determined to get revenge, the Ladbroke Grove group used Facebook that night to recruit "troops and weapons", the Old Bailey heard during a series of trials.

Osoteku, who was taken into care at the age of eight, was at the centre of the conversations and agreed to buy a £3.99 set of kitchen knives from Argos as part of the plan.

The following day her group arrived at Victoria on two number 52 buses at 5.14pm to confront their rivals. Armed with a samurai sword, knives, sharpening steels and metal bars, they charged the group of which Sofyen was a part.

Witnesses said they saw "knives glinting in the air" as the teenager was chased down the escalator into the underground station.

He was pushed or fell down the steps to the ticket hall and was set upon by several youths as he lay defenceless on the floor, all of which was caught on camera. Sofyen was kicked, punched, beaten and stabbed repeatedly in front of dozens of commuters including a priest. His blood was found on at least three knives and a sharpening steel.

Osetoku was caught on CCTV running down the escalator and landing a kick on Sofyen's prone body. She claimed in court that she had just nudged the boy "to see if he was OK".

Three other teenagers, Samson Odegbune and Christopher Omoregie, both 18, and Obi Nwokeh, 19, were convicted of murder during a series of trials last year and jailed for 18 years each last week.

Femi Oderinwale and Adonis Akra, both 18, and Samuel Roberts, 19, were each given 12 years after they were convicted of manslaughter.

Enoch Amoah, 19, and Tyrone Richards, 17, were jailed for seven years after they were found guilty of violent disorder.

Lewis Sinclair, Olawale Olaribigbe and Melvin Mensah, all 18, along with Selassie Ahiaku, 19, received two years each after they pleaded guilty to violent disorder before their trial got underway last September.

Prosecutor Mark Heywood QC described Sofyen's murder as "brutal and merciless".

"Such was their arrogance they carried out that kind of attack in the heart of the capital in a public place," he said.

"That confidence and arrogance came no doubt from the security of acting together as a group in sufficient numbers and with a common purpose.

"It came too from the security of knowing that between them they were so heavily armed as a group that no one individual, small group, police officer or member of station staff could withstand them or stop them or detain them in the course of what they were doing.

"It explains why they did something so truly terrible that many will not, even now, acknowledge that they had any real part to play in it at all.

"It explains why for a trivial slight, they executed a truly terrible revenge."

Osoteku was the last of the attacking group to leave the scene and the prosecution claimed that she had "very great" responsibility for Sofyen's death.

Heywood said: "Victoria Osoteku was one of those who set up and organised the confrontation that led to the death."

Osoteku admitted she was at the scene of the attack but denied being involved. She told the jury she was just following her friends and was shocked to see Sofyen being stabbed.

"I just froze there," she said. "I couldn't believe what I was seeing."