A teenager accused of stabbing a fellow pupil to death outside a classroom was trying to save face with his friends, a court was told this week.
Nottinghamshire crown court heard that the 16-year-old defendant, who cannot be named for legal reasons, showed no emotion when confronted by a teacher after the attack.
A schoolgirl told police in an interview played to the court: "He just looked like he had been told off for chewing in class. He just looked like he didn't care."
Fourteen-year-old Luke Walmsley was stabbed to death on November 4 last year as he chatted to friends after a science lesson at Birkbeck secondary school in North Somercotes, Lincolnshire, the jury was told.
The defendant denies murder.
The trial began last Thursday, when Luke's parents watched in tears as the court was shown CCTV footage of his final steps.
Yvonne Coen QC, prosecuting, said a post-mortem examination revealed the teenager had suffered a single stab wound to the heart. The wound was 4.3 inches deep and the knife had been inserted with "considerable force" at least as far as the hilt.
Ms Coen said the defendant admitted stabbing Luke but denied intending to kill him. The defendant claimed that Luke had said he could beat up the older boy and two of his friends, she said.
"Maybe this was about (the defendant) needing to put young Luke Walmsley in his place and teach him a lesson," she said. The 16-year-old had told other schoolfriends that he would stab Luke weeks before the attack but they did not take him seriously, she said.
Ms Coen told the court that the defendant had threatened another student with the two-and-three-quarter-inch blade earlier in the day after a minor argument over football.
A student broke down in tears when he told the court how he watched his best friend stab Luke with a flick-knife he had given him for his birthday.
The teenager said the defendant looked calm after the attack.
Witnesses said that Luke had done nothing to provoke the older boy, who was standing outside the science class when he walked out.
One said: "He saw (the defendant) and just put his head down. He looked a bit worried. Luke didn't intimidate him or anything. He didn't say anything to him. He didn't glare at him or anything to make (the defendant) do it, so I don't know why he did it."
He said the older boy took a swing at Luke and he heard the knife puncture his skin before spotting the blade as it was pulled out. "I saw the knife coming out of his chest. Some blood squirted just in front of me. I just stood there. I was so shocked I didn't do anything. There was like blood dripping everywhere," he said.
The defendant was then confronted by a teacher and persuaded by another pupil to hand over the knife, the court heard.
He added: "(The defendant) didn't seem that bothered. He didn't look scared or worried."
One of Luke's classmates told police on a videotape played to the court:
"Luke walked out. I walked out after him. (The defendant) got a knife out of his sleeve and stabbed him. I just had to go back in the classroom. I didn't know what to do because I panicked. I just stood there shaking."
He said he later saw Luke collapse and ran over to help him. "He went pale.
There was blood all over his hand, and blood all over the floor," he said.
Sasha Wass QC, for the defence, said that Luke had been passing notes to the defendant's girlfriend the day before the stabbing.
"(The defendant) was rather upset after those notes had gone backwards and forwards between (the girl) and Luke," she said.
The trial continues.