Kieran McGowan was told he is facing a prison sentence after admitting the charge over the death of Kate Whalley.
The teenager was hit by a black Ford Fiesta ST on the A660 Leeds Road, Pool-in-Wharfedale.
The incident happened near to the junction with Old Pool Bank, at about 4.40pm on June 22 last year.
Miss Whalley suffered unsurvivable head injuries but was kept alive at Leeds General Infirmary to allow doctors time to organise recipients for her organs.
McGowan, 23, of Knox Way, Harrogate, pleaded guilty to causing death by dangerous driving when he appeared before Leeds Crown Court today.
Members of Miss Whalley's family were in court for the hearing.
The case was adjourned until June 14 when it is likely that McGowan will be sentenced.
McGowan was granted bail but was made the subject of an interim driving ban.
Judge Guy Kearl, QC, told McGowan that his conviction was "automatically going to lead to a custodial sentence."
The Yorkshire Evening Post reported last year how the family of Miss Whalley said nine of her organs were set to give life to others.
Her father Michael, 54, said Kate had just caught a bus home – after sitting her penultimate A level exam at Harrogate Grammar School – when she was struck by the car.
He said his daughter donated nine organs, including her heart, lungs, liver and kidneys.
He added: “I think it’s wonderful. It just shows the spirit and zest for life that Kate had that she wanted to give life to others.
“It expresses the selfless character she was and what a beautiful young woman she was. “
"She had the most beautiful smile. She was an intelligent, confident, beautiful young woman in every way."