Jodie Chesney, 17, was stabbed to death on 1 March and lost consciousness in her boyfriend’s arms.
Svenson Ong-a-Kwie, 19, and Arron Isaacs, 17, were jailed for murder on Monday
Both had denied being involved in Jodie’s death, with each claiming the other teenager had stabbed her.
Sentencing the pair, Judge Wendy Joseph QC found Ong-a-Kwie had stabbed Jodie himself, and drug-runner Isaacs was his “enthusiastic supporter”.
The judge said the killing resulted from a “tit-for-tat” vendetta of escalating violence between rival drug gangs, and there had been “no true explanation” from the killers over why Jodie was the victim.
Ong-a-Kwie was jailed for life with a minimum term of 26 years, while Isaacs was jailed for at least 18 years.
Isaacs’ name was previously protected by an automatic reporting restriction because of his age, but it was lifted by Judge Joseph after she jailed him.
Only Ong-a-Kwie fitted the description of a tall black male who witnesses saw wielding the knife during the completely silent attack.
Jodie’s friends told the court how she was stabbed seconds after two figures had approached from the darkness, screaming before passing out.
Jodie died before she arrived at hospital from an 18cm deep stab wound to the back, which almost passed through her body.
Judge Joseph said the teenager was the victim of “callous, casual and irresponsible violence” which had shocked the local community “to the core”.
She said the defendants had gone to the park in Harold Hill, east London, for a “planned attack on a rival group” and mistook their target.
“I’m sure that whoever drove the knife almost through Jodie’s body could have intended nothing but death,” the judge added.
She told the court that Jodie was a “bright, warm and loving girl” who “should have led a happy and fulfilled life”.
“Above all those who loved Jodie will never get over her death,” she added.
The victim’s father, who has set up a charity in her name to steer young people away from knife crime, had been celebrating his birthday when police found him to deliver the news of his daughter’s death.
Peter Chesney told the court his life had been “destroyed”, adding: “I have lost the most precious human being I will ever know. I have no idea how I’m going to continue my life or come to terms with the loss.”
Jodie’s boyfriend, Eddie Coyle, said he had been in love with her in a statement read to the court.
“Jodie was funny, silly, she always made fun of me and she had a bright future ahead of her,” he said. “She was full of energy.”
Jodie’s sister, Lucy Chesney, said she had “lost half” of herself.
“We went through everything together and she was always there for me and always putting everyone before herself,” a statement added. “She gave me a type of love I will never feel again.”
Ms Chesney said her sister’s killers had “ripped away a bright future that was destined to make a change to many lives”.
Saying she had been left anxious about leaving her house, she added: “If someone as good and pure as Jodie could be murdered, it could happen to anyone.”
The popular teenager was studying psychology, sociology and photography at Havering Sixth Form College and her friends, family and dog Woody “meant the world” to her, loved ones said.
Jodie, a keen classical pianist and photographer, had achieved the Duke of Edinburgh bronze and silver awards and was just a few weeks away from completing her gold.
She was also an active Scout member and “touched the lives of many other young people” through her volunteer work.
Metropolitan Police investigators admitted they still do not know why Jodie was stabbed.
Police said her killers had gone into the park “purposefully to stab somebody and they didn’t care who it was”.
Investigators believe Ong-a-Kwie, a prominent local drug dealer, was seeking retribution against another person dealing on his turf.
“He went in the park with intent to cause harm to somebody,” Detective Chief Inspector Dave Whellams said. “Quite why it was Jodie we don’t really know.”
Earlier that day, one of her group had unsuccessfully tried to buy £20 of cannabis from Ong-a-Kwie but turned to another local dealer instead.
But Judge Joseph said she “entirely rejected” suggestions that the purchase sparked the stabbing, adding: “The reasons underlying this killing are far more sinister and dangerous than that.”
Police confirmed that Jodie and her friends were smoking cannabis at the time of the stabbing, but said the group were “innocent”.
Det Insp Perry Benton, said: “Jodie was innocent and the group in the park were innocent. If they bought cannabis off one person or another there was never a justification for what they did to her.”
The murder happened after a string of “tit-for-tat” attacks where Ong-a-Kwie had previously been stabbed, and he and Isaacs launched an unprovoked knife attack on a rival over a girl.
Prosecutor Crispin Aylett QC had told jurors they took a “casual approach to violence” in a world where knife crime was “routine”.
CCTV captured the shadowy figures of Ong-a-Kwie and Isaacs disappearing into the park before the sounds of Jodie screaming could be heard.
Manuel Petrovic, a 20-year-old drug dealer, and a 16-year-old boy stayed in a getaway car during the attack and were acquitted of murder earlier this month.
Ong-a-Kwie and the Isaacs were driven away from the scene but later traced and arrested following the discovery of the dumped vehicle.
The court heard Ong-a-Kwie had convictions for possessing and supplying drugs.
He admitted being in breach of a six-week suspended sentence for handling stolen jewellery dating back to October last year.
Isaacs had previous convictions for possessing a screwdriver, actual bodily harm, possession of cannabis as well as aggravated vehicle taking.
Judge Joseph said: “A blameless girl is dead at the hands of those engaged in and those that associate with drug dealing, on the streets which Jodie and other blameless young people must live.
“This death has brought great unease in the community. Those suffering, which spreads much further than Jodie’s friends and family, need and have a right to known and understand how this has come about.”
Additional reporting by PA