The 14-year-old, who cannot be named for legal reasons, said he accidentally stabbed the schoolgirl in the neck in self-defence during a Christmas light switch-on in Liverpool.
He told the court he wanted to “frighten her away” after an altercation in the city centre on 25 November, 2021.
The teenager admitted possessing the knife, which the court heard had a 7.5cm blade, but denied her murder and manslaughter.
The court heard Ava and her friends became involved in an argument with the defendant and three of his friends after the boys recorded Snapchat videos of the group.
Friends of Ava said the boy “grinned” after stabbing her in School Lane and running away.
Ava’s last words to her friends as she lay dying on the ground were “don’t leave me”, the court heard.
The defendant told the jury: “I promise, I didn’t mean to hit her.”
He said that earlier in the evening he heard one of Ava’s group threaten to stab his friend if he did not delete a video of Ava.
The court heard that after the schoolgirl was struck to the neck the defendant ran away, discarded his knife and took off his coat, which was later found in a wheelie bin.
CCTV showed him and his friends in a shop where the defendant took a selfie and the group bought butter, which he said was for crumpets.
He then went to a friend’s home and when his mother contacted him because police wanted to speak to him he told her he was playing a computer game.
The court was told that he had text his mother saying that he was “not going to the cells” after the stabbing.
After he was arrested, just after 10:30pm, he initially told police he had not been in the city centre but in later interviews blamed another boy for the stabbing.
A 20-second clip showing the stabbing was played during the trial, leaving family members in tears in the public gallery.
More than 20 members of Ava White’s family erupted into cheers as the jury returned a verdict of guilty to the charge of murder.
The defendant, who has attention deficit hyperactivity disorder (ADHD), appeared in court via video link. He put his head in his hands as the verdict was read out.
Mrs Justice Yip said the defendant would be sentenced on 11 July.
She says: “There is only one sentence the court can impose. Of course what I need to do is set the minimum term he is required to serve. I will need to have reports for that.”
Speaking after the verdict, Liverpool mayor, Joanne Anderson, said: “Our thoughts are with Ava White’s family and friends on this extremely difficult day.
“The pain and suffering they have been through over the past months is unimaginable.
“We can only hope that the verdict brings them some comfort.”