The five students injured in the blast were rushed to Vajira and Klang hospitals for treatment.
Ministry of Education officials are investigating the cause of the blast.
Ministry permanent secretary Atthaphon Sangkhawasee said officials from his ministry have been sent to the school to determine the cause of the explosion.
He expressed condolences to the relatives of the deceased and victims, saying that the ministry will assist and compensate the victims.
Provincial Education Offices nationwide have been instructed to ensure safety for students, he added.
Weerachai Phutthawong, a lecturer at Kasetsart University's Faculty of Liberal Arts and Science, said the fire extinguisher may have exploded because it was old and damaged.
Fire extinguishers can sustain pressure of about 126 kilos, and any higher pressure can damage them and cause them to explode, he said.
Over filling an old fire extinguisher can also cause it to explode, Weerachai said.
Exposure to sunlight can also increase the pressure of a fighter extinguisher, he added.
Relevant agencies should examine the explosion and ensure that all fire extinguishers and related equipment complies with safety standards.