Melissa Tate was struck by a car in Hillsview Avenue in Kenton, Newcastle, on the evening of Wednesday September 25.
The youngster, who was described as "beautiful" by her family, was rushed to hospital but later died.
Appearing at Newcastle Crown Court on Monday, 24-year-old Connor Marsden admitted causing her death by dangerous driving.
Marsden, wearing a white sweater and appearing via video-link, gave no reaction as he entered the plea, and also admitted failing to stop following the crash.
He will be sentenced at a later date.
Following her death, Melissa's parents, Michael Tate and Kimberley Wilson, said the schoolgirl had saved the lives of sick children after the family decided to donate her organs.
Mr Tate said: "Melissa was a beautiful, smart, funny, cheeky and laid-back girl who had a heart of gold and not a bad bone in her body.
"She could brighten up anyone's day and all she ever wanted to do is help people.
"Even after passing she still went on to help others by donating her organs."
Marsden was told he will have to appear in court in person when he is sentenced on December 12.
Judge Robert Adams told him: "You must understand that these are serious matters you have admitted, and it's inevitable there will be a substantial custodial sentence."
Mark Guiliani, prosecuting, said Marsden was estimated to have been travelling at around 47mph on the 20mph road, but his defence team argue that his speed was actually lower.