A 15-year-old boy who used a hunting knife to stab a fellow pupil at their school during a lunch break has been found guilty of murder.

Harvey Willgoose, also 15, was stabbed to death in front of other children who fled “in fear and panic” after witnessing the attack in a courtyard at All Saints Catholic high school in Sheffield on 3 February. Some ran to a nearby storage cupboard which they locked and barricaded.

The defendant, who cannot be named because of his age, admitted manslaughter but denied murder, saying he “snapped” after a long period of bullying and did not remember what happened.

However, during a month-long trial at Sheffield crown court, the prosecution said the boy “wanted to show he was hard” and “knew exactly what he was doing”.

On Friday a jury found him guilty of murder by a majority of 11 to one after deliberating for more than 14 hours.

The court heard the stabbing was probably an “act of retribution”, to “get back at Harvey for something”.

Harvey’s mother, Caroline Willgoose, burst into tears after the verdict was delivered. She held her husband Mark’s hand and said “yes”, but there was no reaction from the defendant.

In a statement read outside court Harvey’s sister, Sophie Willgoose, said her brother had been “full of life” and “had a unique ability to bring people together”.

The trial heard the two boys fell out several days before Harvey’s death after taking opposite sides in a dispute between two other boys.

Prosecutors said the defendant had an “unhealthy” interest in weapons and had photos on his phone of him posing with other hunting-style knives and a machete.

Richard Thyne KC, prosecuting, told jurors the defendant “had a significant history of becoming angry and using violence at school”. He said records from a previous school “show a pattern of behaviour ranging from defiance, through to the use of physical violence towards other pupils and property”.

Thyne said the defendant had researched rage rooms and, just over a week before the fatal stabbing, searched “waiting for someone to swing so I can let out my anger”.

He told jurors that on the day of the stabbing, CCTV footage showed him trying to provoke Harvey, who remained “peaceful”.

The court heard Harvey told his friends that the defendant had been “acting like he had a knife” under his jumper in their science lesson that morning, but thought he was bluffing.

Jurors were told Harvey was happily talking to his friends as the lunch break started, when the defendant approached him. One teenage witness said there was an altercation and the defendant punched and pushed Harvey, causing him to stumble backwards, before pulling out a knife and stabbing him.

The defendant told the trial that he was the victim of racist bullying and taunts about a medical condition. It meant he got angry quickly and “couldn’t control it”, he said.

The boy told the jury his mother had mental health problems and his father, who hit him, was often not there. He told the court he took the knife to school because he thought he was going to get hurt that day.

Gul Nawaz Hussain KC, defending, told jurors: “Tragically, Harvey was a combination of being the final straw that broke [the defendant] and the unintended face of a series of threats of violence and bullying he had suffered in recent months.

“We say he suffered a loss of control which resulted in horrific and tragic consequences.”

Responding to the guilty verdict, South Yorkshire’s metro mayor, Oliver Coppard, said it brought some justice for Harvey, “though it cannot undo the pain his family has suffered”.

Coppard added: “He was a young boy with a bright future, and his death, especially in a school, has deeply affected our region.

“Harvey was a young man full of potential, whose life has been lost to violence, while another young man now faces a future behind bars. It’s a stark reminder of the devastating consequences of knife crime and of the urgent need to invest in prevention, education and early support to protect young lives.”

Louise Haigh, the MP for Sheffield Heeley, said: “All of Sheffield will be thinking of Harvey’s family and all those who loved him today.

“An independent review will now take place into the circumstances that led to Harvey’s death, and it is absolutely essential that all questions are answered and that any and all lessons must be learned.”

DI Joe Hackworthy, the deputy senior investigating officer for South Yorkshire police, said: “This trial has been a heartbreaking reminder of the devastation knife crime causes in our communities.

“It tears lives and families apart, and that has never been more evident than in this tragic case.”

Following the verdict the judge, Mrs Justice Ellenbogen, told the defendant he would probably be sentenced in October. She will also consider an application to lift the defendant’s anonymity.