A schoolboy was killed and 10 other pupils were injured when a fire extinguisher exploded at a prestigious school in Bangkok today.

Khumthong Premmanee, 17, was among classmates watching a fire drill at the Rajavinit Mathayom School when the carbon dioxide canister blew up during the safety demonstration at around 11 am.

The huge metal projectile launched from the blast ripped through the boy's body killing him instantly and injured at least 10 other pupils who were watching.

Officials said they were investigating who organised the fire drill and what caused the explosion. They believe the extinguisher had been torn and reused without a safety valve before it heated up in the sun, with temperatures reaching close to 40 degrees Celsius.

Police Lieutenant General Thiti Saengsawang, Metropolitan Bureau Commissioner said the the CO2 fire extinguisher was being used in a safety demonstration when it exploded.

He said: 'The explosion resulted in the forceful ejection of the tank, hitting a student sitting in front of a shrine, leading to an instant fatality.

'Forensics police have gathered evidence at the scene, examined surveillance camera footage, and the student's body is currently awaiting the autopsy results.

'Officers are interrogating seven trainers, as well as teachers and students who witnessed the incident.'

Investigators believe a 'technical difficulty' occurred as participants were using the fire extinguisher, causing its pressure to become unregulated and rise, leading to the explosion.

The canister was also old and had prolonged exposure to sunlight prior to the drill, they said. The school has closed its gates as forensics officers collected evidence from the scene. Police said investigations are ongoing.

Officers are investigating three of the safety trainers on suspicion of 'negligence resulting in the death of another person'.