The brutality of the crime moved the Judge, who in an unprecedented step allowed Joshua Davies, 16, to be named despite being a juvenile, saying it was in public interest to expose the brute whose horrific crime won him a breakfast.
The jury at Swansea Crown Court heard how Davies, a year ago lured Rebecca Aylward, 15, to a secluded wood and then used a rock, the size of a rugby ball to smash her skull with at least six brutal blows to the back of the head.
The boy just 15 when he turned killer and now 16, wept as he was remanded in custody by Justice Lloyd Jones to be sentenced later but was told to expect an "indefinite" term, the Sun reported.
The jury convicted him of murder by a majority verdict after hearing that Davies texted a friend to ask: "What would you do if I actually did kill her?" His friend mistook it for the latest in a long line of jokes about murdering Rebecca and replied: "Oh, I would buy you breakfast."
The schoolgirl's body was found face down in the rain, the following day of the murder after one of Davies's friends contacted police and led them to the scene.
Davies, who was said to have had a "love-hate" relationship with Rebecca, had calmly discussed the various ways he could kill her with a group of friends whom he met for breakfast every Saturday morning in Aberkenfig, South Wales, the jury heard.
One school friend said he was "obsessed" with killing people. After his arrest, police found a concoction of poisonous foxgloves which he had planned to slip into Rebecca's drink. The judge adjourned sentencing for psychiatric reports.