This is the tragic moment a fire extinguisher exploded killing a pupil during a fire drill at a school in Bangkok this morning.

Khumthong Premmanee, 17, was among classmates taking part in the Rajavinit Mathayom School's annual fire safety exercises when the CO2-type canister blew up in front of him on June 23 at around 11am.

Footage shows the officers from the Samsen Fire Station demonstrating how to use an extinguisher, with a cluster of the red cylinders behind them baking under the sun.

Suddenly, a loud bang jolted the pupils when one of the containers exploded. The blast struck Khumthong, flinging him around 30 feet away.

The boy's body had been disfigured with some of his organs ejected when the shockwave and shrapnel hit his torso. At least 10 other pupils were also hurt, investigators said.

Speaking today, Police Lieutenant General Thiti Saengsawang, Metropolitan Bureau Commissioner, said: 'While it was about to be used for demonstration purposes, the CO2 fire extinguisher located behind the trainers exploded.

'The explosion hit a student sitting in front of a shrine, propelling his body approximately nine to 10 metres away. He died instantly. The deceased student was not involved in the demonstration but was seated away from the demo area.

'By 1 pm, the injury count has risen to 10, with six individuals receiving medical treatment at the BMA General Hospital and the Vajira Hospital. The other four were under observation for their injuries.'

Forensics officers said the fire extinguisher had been re-used without a safety valve. It was refilled with gas and had been left sitting under the sun for a prolonged period, causing pressure build-up until it finally exploded, they said.

Police were gathering evidence and CCTV footage from the school, and will be questioning the witnesses, including the trainers involved. Khumthong will undergo a post-mortem examination, officers said.

Police Lieutenant General Thiti said three trainers have been charged with negligence resulting in another person's death.

School staff have closed the gates while investigations were ongoing, but said Khumthong's grieving family were allowed inside to talk with the authorities.