Schoolboy Harvey Willgoose was stabbed to death with such force by a fellow pupil the blade sliced through one of his ribs, the pathologist who carried out the postmortem Harvey, 15, collapsed and died after a snapchat spat with a another student exploded in deadly violence during a lunch break at All Saints Catholic High School in Sheffield.

The killer admits stabbing Harvey through the heart with a 13cm hunting knife but claims it was manslaughter not murder. A jury at Sheffield Crown Court has been shown chilling CCTV footage of the stabbing on February 3, which resulted in other pupils fleeing in what prosecutors called “fear and panic”. Today Home Office pathologist Dr Philip Lumb told how he found an 8cm deep stab wound in the dead teenager’s chest. Dr Lumb said this injury had completely severed his 5th rib, nicked the rib below and gone into one of the main pumping chambers of his heart.

The pathologist said police showed him a 13cm long knife before he conducted a post-mortem examination on Harvey, and he agreed that this weapon was capable of causing the fatal injury.

He explained to the jury that pathologists use a crude scale of “mild, moderate or severe” to grade the level of force needed to inflict an injury.

Dr Lumb said: “In this case, we have the full thickness of the bone of a rib being cut through.

“Bone is a very hard substance. It’s very difficult to cut, even when using a sharp knife.

“And that is a marker of the use of severe force.”

Prosecutors have told the jury how the CCTV footage shows that Harvey appears to put his hand on the defendant’s right arm before the defendant “takes a knife out of his left pocket, passes it across into his right hand, and then stabs twice at Harvey’s torso”.

Seconds later, the video shows Harvey running towards the defendant, who then advances for a second time, “bouncing on his toes, still brandishing the knife”, the court heard.

Richard Thyne KC, prosecuting, said “other pupils fled in fear and panic” as the defendant went into the dining hall still holding the knife.

The boy, who cannot be named, has also admitted possession of a knife on school premises.

On Monday the jury heard how Harvey told his dad he “wasn’t going to school where people had knives” days before he was fatally stabbed in the heart following a social media spat with the fellow student.

After being disarmed by teachers the knife-wielding youngster told the school headteacher: “I’m not right in the head” after he had smuggled his weapon into class.

Days before his death Harvey skipped class following a knife incident at the school involving other pupils, text messaging his dad to tell him: “Am not going in that school while people have knives”.

It is alleged the previous incident led to Harvey and the defendant falling out in a Snapchat group, with each siding with one of the boys involved in the initial dispute, who had been suspended.

Opening the case Richard Thyne KC, prosecuting, told how after being disarmed the killer told All Saints’ head Sean Pender: “I’m not right in the head. My mum doesn’t look after me right. I’ve stabbed him.”

The prosecutor said the boy confirmed to Mr Pender he was referring to Harvey, and that he had stabbed him once or twice.

Mr Thyne said: “Whilst waiting for the emergency services to arrive, (the defendant) also told Mr Pender that he was carrying the knife for protection.”

The prosecutor revealed the altercation happened in a school courtyard just as the lunch break was starting.

He played the chilling CCTV footage to the jurors, telling them: “It is shocking, but it is necessary to play it.”

Mr Thyne said the prosecution case is that the CCTV shows Harvey appearing to put his left hand on the defendant’s right arm before the defendant “takes a knife out of his left pocket, passes it across into his right hand, and then stabs twice at Harvey’s torso”.

He said the defendant then advances towards Harvey, who backs away across the courtyard, before the boy “returns towards where the incident began, gesturing towards Harvey with his knife, and appearing to shout at Harvey”.

The video shows Harvey running towards the defendant, who then advances for a second time, “bouncing on his toes, still brandishing the knife”, the prosecutor said, adding that then “Harvey backs away”.

Mr Thyne said “other pupils fled in fear and panic” as the defendant went into the dining hall still gripping the knife.

Staff members Carolyn Siddall and Rachel Hobkirk approached the boy as he “was dancing around on his toes and waving the knife around, although by this stage he seemed to be saying ‘I’m not going to hurt anyone'”, the prosecutor said.

He added: “They told him to put the knife down but he did not do so.”

He told the jury assistant head Morgan Davis arrived and “found the defendant still waving the knife around”.

As Mr Davis told him to hand over the knife, the defendant was saying to him: “You know I can’t control it,” which Mr Thyne said the teacher took to be a reference to his anger issues, given previous incidents of violent behaviour at school.

The prosecutor said: “Mr Davis held his hand out and took the knife from (the defendant).

“At the same time the headteacher, Mr Pender, placed his arm around (the defendant)’s shoulder and took him along the corridor to his office.”

Mr Thyne earlier told the jury how Harvey was stabbed in the heart with a hunting knife which had a 13cm, serrated-edged blade.

He said the defendant “admits that he stabbed Harvey causing his death” and “also admits that the stabbing was not carried out in lawful self-defence.”

The trial continues.