Logan MacPhail, now 17, knifed Holly Newton 36 times in an alleyway after stalking her for an hour through a town centre last January.
He denied murder but admitted manslaughter, claiming he blacked out, but was convicted following a trial at Newcastle Crown Court.
Earlier this month, the judge lifted a reporting restriction preventing the media from identifying the teenager.
Holly's grieving mother, Micala Trussler, made a moving victim statement at the start of a two-day sentencing hearing yesterday, watched by MacPhail via a videolink.
Enormity
The mother of four said she raced to the scene of the tea-time attack in Hexham, Northumberland, frightened of what she would find - but was met by a more horrific scenario than she had imagined.
Micala told the hearing: "The enormity and severity cannot be put into words.
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