Judge Michael Coombe said Danny Westmacott was the victim of a "cowardly and vicious murder" carried out "by way of execution", and he condemned the wall of silence which had surrounded the investigation into the stabbing.
He urged a change in the law to empower courts to mete out "condign" or deserved punishments to young witnesses to crimes who refuse to co- operate with police and the courts.
The judge ordered Noel Sayles, 16, and Jason Honeyghon, 17, who had both denied the murder, to be detained at Her Majesty's pleasure for the murder.
The teenager was stabbed in revenge after winning a fight over "a stupid little incident" at a McDonald's restaurant in Edmonton, north London, in February last year. A children's party was in progress - and youngsters were forced "to witness that savage murder", said Judge Coombe.
Danny Westmacott, a student, was a regular visitor to McDonald's restaurant in Edmonton. On the night of the murder, another teenage customer, Noel Sayles, had the impression Danny and his friend were staring at him and the two girls he was with.
"It was one of those stupid little incidents which probably occurs 100 times a day all over the country," said David Calvert-Smith QC, prosecuting.
A fight broke out between the two in which Danny wrestled the other boy to the ground and began kicking him. After the fight was broken up, Sayles decided on revenge, Mr Calvert-Smith said.
Sayles, then 15, telephoned a friend and when he returned to the girls he was heard to say "watch when my brethren bring the borrer", which translated to "watch what happens when my friends bring the knife".
Sayles returned to the restaurant with three others. He and Jason Honeyghon "went straight up to the two boys and attacked them. They knew who to attack," Mr Calvert-Smith said.
Danny was fatally stabbed in the back.