A 15-year-old boy who died trying to protect his friends is unlikely to have seen the weapon that killed him, the Old Bailey heard.
Kasey Gordon 15, died in Tottenham, north London, in January 2011 after a row with Serif Aslan over a friend's girlfriend.
The Old Bailey heard that Aslan, 34, also of Tottenham, had tried to woo a girl who was with a group of schoolfriends. His attempts led to a row with the group. He went home, returned to the scene with a knife and attacked them. Kasey died in the confrontation.
Three other children, aged between 14 and 16 and all from the Park View Academy school in Haringey, were treated for stab wounds.
The Old Bailey heard that despite Aslan being warned against carrying a knife in public he carried a concealed blade after he was first rebuffed by the group and attacked them.
Richard Horwell QC, prosecuting, said: "This main part of the incident lasted about 30 seconds and when it was over the defendant calmly walked away. He left behind a scene of considerable carnage.
"The knife that he had carried, the knife he had been told not to carry, was used by him to devastating effect.
"Even though this event had been so short, the defendant had stabbed three schoolboys and had injured a fourth," he said.
He added that Kasey was unlikely to have seen the murder weapon.
Aslan denies murder, wounding with intent to cause grievous bodily harm, and causing actual bodily harm.
He claims that he was acting in self-defence after one of the boys punched him for making an inappropriate comment to his partner.
The trial continues.