Joshua Davies, 16, 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.
Davies then went to a friend’s house where he drank tea and bragged about what he had done. That evening, he created an alibi by posting messages on social networking site Facebook claiming he was “chilling out with friends” while watching Strictly Come Dancing.
After Rebecca’s family had reported her missing, he used the website to feign concern.
In one exchange he wrote: “I feel sorry for her mother.” When asked why, he replied: “Well if I was a parent I’d be worried if my daughter was missing.”
Davies showed no emotion as the jury delivered its 10-2 majority verdict, which was met with brief cheers from Rebecca’s family.
Mr Justice Lloyd Jones lifted an anonymity order because of the “strong public interest in open justice.”
He said: “This is a crime that has affected a small enclosed community. It is right that the public should know that there has been a conviction and who is convicted.”
Davies, who was said to have had a “love-hate” relationship with Rebecca Aylward, 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.
Over a cup of tea and a full English, he disclosed that he was considering poisoning, drowning or pushing her off a cliff, bragging about how he would “get away with it” and how he could “cover up” the killing.
After one such meeting Davies sent a text to a friend asking: “What would you do if I actually did kill her?” The friend replied: “Oh, I would buy you breakfast.”
On the day of the murder, last October, Davies met the group as normal before leaving to meet Rebecca. Before he left, he said: “The time has come.”