A 14-year-old schoolboy opened fire Friday morning at Debsirin Nonthaburi School in Nonthaburi province northwest 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 hospitalised, with seven of them in critical condition on Saturday, according to the health ministry.
Most of those wounded 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.
Emergency workers brought out a coffin and loaded it onto an ambulance, which will transport them to their hometown for funeral rites.
Wiroon Supasingsiripreecha, head of the institute, said most of the victims suffered a single fatal gunshot wound to a vital area, including the chest or head.
He said the gunshot wound found on the alleged shooter was consistent with the weapon he had used.
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 classes would be suspended between August 10 and August 14.
On Saturday morning, the school was quiet with only a minor police presence at the entrance, with a few mourners laying flowers at the gate.
Charin Siriananchai brought his two sons, ages nine and six, to pay their respects.
He said while his sons did not study here, he lived in the neighbourhood and drove 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 Southeast Asian neighbours.
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 relatively high in Asia.
Only the Philippines is higher in Southeast Asia.
While shootings involving indiscriminate killings are not common, the country has seen an uptick 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 wounded two others.
A 14-year-old boy was accused of a 2023 shooting at a major Bangkok shopping centre.
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