Those killed at the school were all teachers and staff and police said the 14-year-old shooter, a student, fired at least 26 bullets using a gun that belonged to his grandfather.
An additional 34 rounds were found, police said.
Students streamed out of the Debsirin Nonthaburi School, on the capital's northwestern outskirts, on Friday as the wounded were loaded into ambulances and teachers hugged each other and cried.
Police said 23 people were wounded - nine critically - in the incident, the latest in a succession of high-profile shootings in recent years in Thailand, which has by far the largest number of guns in civilian possession in Southeast Asia, and among the most per capita in Asia, according to a 2017 estimate by the Small Arms Survey.
One 18-year-old student told Reuters he initially thought there were firecrackers going off or someone banging an object.
"I didn't think it was a gun at first," he said.
"There were many shots: bang, bang, bang. Then it went quiet. Then it started again."
Education Minister Prasert Jantararuangtong told Reuters that both teachers and students were among the casualties.
The motive for the shooting was being investigated, he told reporters.
It is the second school shooting in Thailand in 2026 after a teacher died and a student was injured in the south of the country in February.
Emergency worker Kiatikhun Verapongpradith, 47, described arriving as the shooting was ongoing and his team treated students with injuries in the back, chest and arms.
They found a male teacher lying dead on an upper floor of the school, and in another room they found a female teacher with wounds in her chest and arm.
He said he was called to treat the teenage shooter after hearing a final shot.
"We rushed up and found that the perpetrator had shot himself in the right side of the head and collapsed. When we got up there, we checked his pulse, he still had one, so we started CPR."
In the 2025 academic year, the school had an enrolment of about 3100 students and 147 teachers, according to district authorities.
"This is terrible for something like this to happen," Prime Minister Anutin Charnvirakul told reporters.
"I'm sad for those who died and sad for something like this to happen in our country."
Thailand's worst mass killing by a single perpetrator in its recent history took place in 2022, when a sacked former policeman ran amok in a three-hour gun-and-knife attack in the northeast that killed 36 people including his wife, son and 22 children stabbed as they slept at a daycare centre.
In 2020, a soldier killed 29 people in a shooting rampage in Nakhon Ratchasima that included a shopping mall, while in July 2025, a gunman killed five people at a Bangkok market, among them security guards and a vendor, before turning the gun on himself.
In 2023, a 14-year-old killed two people and wounded five others at a luxury Bangkok shopping centre, using a modified handgun that police said was likely purchased online.
After the 2023 shooting, Anutin, who was then serving as interior minister, ordered a raft of gun control measures, including a short-term ban on new licenses, import restrictions, and banning of people under 20 years from shooting ranges.
Longer-term measures also included a medical certificate assessing the mental health and psychological condition of gun owners as well as a specified validity period for some gun licences.
Thailand had an estimated 10.3 million firearms held by civilians, equating to about 15 guns per 100 residents, the Small Arms Survey estimated.
with AP
