A 14-year-old schoolboy opened fire at around 10am local time (4am BST) on Friday at Debsirin Nonthaburi School in Nonthaburi province, north-west of Bangkok, before taking his own life.
He also appeared to have killed his two grandparents at the family home earlier in the morning, officials said.
Sixteen people remain in hospital, with seven of them in a critical condition on Saturday, according to the health ministry. Most of those injured were between 12 and 14 years old.
All eight bodies – five school staff members, the alleged shooter and his grandparents – were collected by early on Saturday afternoon from Bangkok’s Institute of Forensic Medicine.
Wiroon Supasingsiripreecha, head of the institute, said most of the victims had suffered a single fatal gunshot wound.
He said the gunshot wound found on the alleged shooter was consistent with the weapon he had used. More detailed forensic findings, including toxicology results, are expected later.
Officials said the gun, described as small and compact, was legally registered to the boy’s grandfather.
There were reports that the boy, who lived with his grandparents, had shown signs of stress connected to school, although some of his relatives told Thai media that they were unaware of any troubles he might have been facing.
The school said all all classes would be suspended between August 10 and 14. School staff were instructed to work from home during that time.
On Saturday morning, the school was quiet with only a minor police presence at the entrance. A few mourners came to lay flowers at the gate.
Charin Siriananchai brought his two sons, aged nine and six, to pay their respects. He said while his sons do not study here, he lives in the area and drives past this school almost every day, often stopping to allow students to cross the road.
“Everyone was saddened and shocked, because it was unexpected that this would happen in Thailand, and also it’s quite a great loss,” he said.
Some parents also came with their children to collect their belongings left behind after the shooting. A father, who declined to identify himself, said his son, who was at the school during the shooting, was still in shock and could not sleep alone.
Thailand has one of the highest rates of gun ownership in Asia, second only to Pakistan and far surpassing its south-east Asian neighbours.
Data collected in 2017 by the groups Small Arms Survey and GunPolicy.org in their last extensive international study found civilian gun ownership in Thailand was about 15.1 guns per 100 people, compared with less than one per 100 in Malaysia.
Thailand had 3.49 deaths by firearms per 100,000 people, according to 2023 statistics published by World Population Review.
That puts it well behind parts of Latin America and the Caribbean, but is relatively high in Asia. Only the Philippines is higher in south-east Asia.
While shootings involving indiscriminate killings are not common, the country has seen a rise in high-profile shooting deaths in recent years.
In February, a 17-year-old stole a gun from police and opened fire at a public high school in southern Thailand, briefly taking people hostage in a two-hour attack that killed one person and injured two others.
A 14-year-old boy was accused of a 2023 shooting at a major Bangkok shopping centre.