First he shot dead his two grandparents, using his grandfather's handgun, then went to a secondary school in Nonthaburi province, bordering Bangkok, and killed five school staff, national police said in a statement.

The 14-year-old perpetrator also died, the statement said, with a police official telling AFP he had killed himself.

Officials earlier gave a higher toll of eight dead, which had included the suspect, and initially said students were among the victims.

Dozens of police and emergency responders gathered outside the cordoned-off Debsirin Nonthaburi School, AFP journalists saw, while parents waited to pick up their children and tearful students and staff comforted each other outside.

"I was afraid that I would die, and I was scared that I wouldn't be able to pursue my dreams," said Pawarisa Maylissa, a grade 12 student who hopes to attend medical school.

"I heard many gunshots very loudly because it seemed the suspect was on the floor directly above us," she told AFP.

Prime Minister Anutin Charnvirakul told reporters at the scene that "police interrogated his close friend who said the suspect had been under stress" related to his schooling.

"His actions also suggested that he had clearly planned" the assault, Anutin added.

"The gun was a registered firearm and was kept at a house, but somehow he gained access to it."

The prime minister said more than 30 people were injured, with around nine in critical condition.

The shooter was identified by local media as a student of the school.