Two teenagers have been jailed for life for the “callous, casual and irresponsible” murder of 17-year-old Jodie Chesney.

Drug dealer Svenson Ong-a-Kwie, 19, and his 17-year-old runner Arron Isaacs had been looking to take revenge on rivals when they killed Jodie by mistake, the Old Bailey heard.

Jodie, who was a Scout and described as a “girl next door”, had been smoking cannabis and listening to music with friends in a park in Romford in March when she was stabbed in the back.

She screamed and collapsed in the arms of her boyfriend, Eddie Coyle, 18, as her attackers, who did not know her, made off in a fellow drug dealer’s car. Days later, they were arrested together as they fled from a house.

The defendants denied Jodie’s murder, each blaming the other for inflicting a wound that almost went through her torso. But prosecutor Crispin Aylett QC told jurors that both were responsible for the killing, amid incidents of “casual violence” in the drug-dealing world. The jury deliberated for less than six hours last week to find them guilty of murder.

Jodie Chesney was relaxing with friends when she was fatally stabbed. Photograph: Metropolitan Police/PA

Judge Wendy Joseph QC handed Ong-a-Kwie a life sentence with a minimum of 26 years. She sentenced Isaacs to be detained for at least 18 years. Two co-defendants who were in car with Ong-a-Kwie and Isaacs were cleared of murder.

The judge said Jodie’s death led to a “world of anguish” for her family, which came amid “tit for tat” incidents. Jodie was the victim of “callous, casual and irresponsible violence” that had shocked the community “to the core”, she said.

On the evidence, she found Ong-a-Kwie wielded the knife and Isaacs was his “enthusiastic supporter”.

The judge lamented the impact of criminals that had “carved up areas of the capital of this country”. She added: “The dangers this brings to decent law-abiding members of the public is graphically spelled out in this case.”

The court heard moving tributes to Jodie and the “ripple effect” her murder had among friends, family and the wider community.

Coyle has been left with post-traumatic stress from witnessing her murder. He said: “Jodie was funny, silly, she always made fun of me and she had a bright future ahead of her. She was full of energy and was always out doing something. We had been going out for three months.

“I’ve never lost anyone before and for the first funeral I’ve gone to to be my own girlfriend’s is incredibly hard. I loved her.”

Jodie’s father, Peter Chesney, who gave up a job in the City after her death, said: “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.”

Her sister, Lucy, said: “Jodie will be greatly missed and the people who caused such tragedy to a whole family should hang their head in shame. You have ripped away a bright future that was destined to make a change to many lives.”

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 and aggravated vehicle-taking.

Speaking outside the Old Bailey after sentencing, DCI Dave Whellams said he was “pleased and proud” of the convictions. He also defended the Metropolitan police’s decision to charge the two men who were cleared of murder. He said: “It is very important that all four people that were involved in the events that unfolded that night were put into the dock and have the opportunity to give their account to the jury.”

His colleague on the investigation, DI Perry Benton, said it was one of the hardest investigations he had ever had to deal with. “It felt the world was waiting for answers,” he said. “It was horrific and horrendous and that’s going to stay with everyone for a very long time.”

Asked what more could be done to tackle knife crime, he said: “It’s not just the police’s problem, it is a public health approach that’s required. Hopefully these sentences will be a deterrent for others that may think about carrying knives and knowing the injuries and the lives that they devastate.”

Speaking on behalf of Jodie’s family, her uncle Terry Chesney thanked the police for their work.

“She will never be back but this was some justice,” he said.