Hannah Bonser has been jailed for life after being found guilty of the murder of 13-year-old Casey Kearney. She has been told she will serve a minimum of 22 years in prison.
Casey was stabbed in a random attack in Elmfield Park, Doncaster, South Yorkshire, on Valentine's Day.
The judge, Mr Justice Cranston, told Sheffield Crown Court Casey's family had reacted with "enormous dignity" as they listened to the details of her death.
He added: "Casey did not deserve this. She had done nothing wrong."