The boy, now 17, had admitted manslaughter but denied murdering 15-year-old Holly in Hexham, Northumberland, in January 2023.
After a five-week trial at Newcastle crown court the boy, who was 16 at the time and cannot be identified, was found guilty of murder.
Holly, from Haltwhistle, was murdered after she had finished her day at school in Hexham and was walking round shops with friends. David Brooke KC, prosecuting, said the defendant was following Holly and her friends from a distance.
"It is quite clear he was being careful not to be seen by them for the next 45 minutes or so," Brooke said.
The boy made himself known to Holly, who was not happy to see him, the court heard. She agreed to speak to him down an alleyway.
"What Holly didn't know was that (the defendant) had brought a knife with him," Brooke said.
In less than a minute, the defendant inflicted 36 knife wounds on Holly, stabbing her 12 times and slashing her 19 times. She suffered a further five defensive injuries to her hands. A 16-year-old boy who was with Holly tried to help and was also stabbed.
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