The 17-year-old grabbed the schoolgirl as she walked along a street and dragged her to a nearby riverbank in Exmouth, Devon, on October 4, 2018.
He restrained the girl around the neck after snatching her and then raped her after she fell unconscious next to the water.
A passerby came to her aid after hearing her screaming for help.
The boy, who was 16 at the time of the attack and cannot be named for legal reasons, previously pleaded guilty to rape on the first day of a retrial at Bristol Crown Court on July 22.
He had earlier admitted choking and sexually assaulting the girl.
In March, a jury at the first trial acquitted the boy of attempting to murder the girl.
Jurors could not reach a decision on charges of rape and choking with intent to commit rape.
During the trial the jury heard how the boy grabbed the girl, who he did not know, as she walked home from school.
He restrained her around the neck and walked her to a nearby riverbank, where he raped her after she fell unconscious.
The boy claimed he checked the girl had a pulse and waited until she woke up before leaving the area.
A passerby heard the girl yelling from the stream and went to her aid, with police called a short time later.
During his evidence in the trial, the boy said he carried out the attack because of "extraordinary emotions".
He claimed he was angry, scared, depressed, sad and lonely at the time.
"I just wanted someone else, just anybody, to feel at least some of the way I was feeling," he told the jury.
The boy admitted the charge of rape on the first day of his re-trial in July.
Mrs Justice May jailed the boy, who admitted charges of rape, choking the girl with intent to commit sexual assault and sexual assault, for seven years and two months.
She also imposed an extended sentence of eight years on licence, which the boy will serve after his release from prison.
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