Ava was fatally stabbed in the city centre on November 25 last year.
More than 20 members of her family in the public gallery erupted into cheers as the jury returned a verdict of guilty to the charge of murder.
The defendant, on videolink, put his head in his hands.
The jury was out for two hours and eight minutes before returning the verdict.
Mrs Justice Yip said the defendant would be sentenced at a later date.
Ava White
Merseyside Police
The 14-year-old, who cannot be named for legal reasons, had claimed he accidentally stabbed the schoolgirl in the neck in self-defence.
He told the court he wanted to “frighten her away” after an altercation in Liverpool city centre on the evening of November 25 last year.
He admitted possessing the knife, which the court heard had a 7.5cm blade, but denied her murder and manslaughter.
During the trial, which lasted just over two weeks, 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.
Ava White
Merseyside Police
Friends of Ava said the boy “grinned” after stabbing her in School Lane and running away.
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 Ava 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.
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.