A 15-year-old boy was killed when a homemade gun was fired in a classroom at a Nonthaburi school, in an incident students and teachers initially thought was a computer "explosion".

The incident took place Thursday afternoon in a computer class at Wat Lad Pla Duk School.

The student, later identified as Noppasil Ngamsud, sustained serious injuries to his face, police said. Medics rushed to the scene but were unable to save his life.

Initially, Sasina Rakpan, a computer teacher, told police an "explosion" occurred after a group of Grade 9 students sat down for their class. She said she immediately ran to check and found one bloodied student lying unconscious.

One 15-year-old student, who said he was sitting next to the victim, said the explosion occurred about 10 minutes after Noppasil turned off his computer. He said it left him with ringing in his ears. Another classmate who was sitting on the left side of the victim was hit by an object and was slightly hurt, he said.

However, police were not satisfied with the statements and ordered investigators to check the wound of the dead student, where they found traces of a bullet that struck his eye.

During the subsequent questioning, one student later confessed to taking a pistol to school "to clear up some problems with his friend", according to a police source.

However, police said on Friday that a student had been showing off his gun and dropped it on the keyboard. The gun went off and the bullet killed his friend.

"Investigators believe that the offender showed his gun to a friend. Then the gun fell on a keyboard, which  broke, and accidentally discharged and the boy was fatally shot. That's all," police said.

Police questioned eight witnesses and found no conflict, police added.

The homemade gun believed to have accidentally discharged and killed a student. (Photo supplied)