The girl appeared by video link at Leeds Crown Court where she pleaded guilty to manslaughter by diminished responsibility.
Katie was found with severe lacerations to her neck and chest on a playing field in York in January and died later in hospital.
The dark-haired girl appeared on the video link sat next to a solicitor and wearing a black hoodie.
Her solicitor confirmed her name when asked by the judge, Mr Justice Soole.
Nicholas Johnson QC, defending, asked the court if the charge of murder could be put to the girl again and she wrote her plea on a piece of paper.
Her solicitor told the court: "I can confirm she has indicated not guilty to murder but guilty to manslaughter."
Graham Reeds QC, prosecuting, said: "We are going to accept that plea of manslaughter by diminished responsibility."