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

The student, 17, was among classmates taking part in a fire drill at the Rajavinit Mathayom School when the carbon dioxide canister blew up during a fire extinguishing exercise at around 11 am.

The blast killed the boy and injured at least five other pupils, local media reported.

Officials said they were investigating who organised the fire drill and what caused the explosion.

The local rescue team 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.