Luke Walmsley was stabbed in the heart

Science teacher Carol Mortimer said she heard a scuffle outside her class at Birkbeck Secondary School in Lincs.

As he pulled the bloodstained blade from behind his back, a slight smile crossed the boy's face, she said.

The teenager, who cannot be named, has denied murdering Luke Walmsley, 14.

Other teachers from the school in North Somercotes, made statements to police in which they described the struggle to resuscitate Luke on 4 January last year.

Luke was struggling to breathe and very pale David Allison

Art teacher David Allison said: "My initial thought was that there had been a scuffle.

"I looked at Luke's face. At this time I realised the incident was serious.

"Luke was struggling to breathe and very pale. The ambulance seemed to take quite a while to arrive."

A consultant surgeon at Grimsby's Diana, Princess of Wales Hospital said he found Luke with a single stab wound to the chest.

He told Nottingham Crown Court it seemed "a significant force was applied to the instrument that caused the injury in the first place".

Yvonne Coen QC, prosecuting, asked him: "Is it your opinion that the damage to the sternum found was caused by the stab wound rather than any medical intervention?"

He replied: "It is."

The trial continues.