Those killed at the school on Friday were teachers and staff, and police said the 14-year-old shooter fired at least 26 bullets using a gun that belonged to his grandfather.
An additional 34 rounds were found.
Students streamed out of the Debsirin Nonthaburi School, on the capital's northwestern outskirts, as the wounded were loaded on to ambulances and teachers hugged each other and cried.
Police said 23 people were wounded in the incident, the latest in a succession of mass 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."
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.
"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."
After consultations with officials, Anutin said the gunman had suffered from stress related to his studies and lived with his grandparents, whose bodies police discovered when they visited their home after the school shooting.
He said nine of those wounded were in critical condition.
He also vowed to introduce a new firearms control law.
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 teacher dead on an upper floor of the school, and in another room a female teacher with chest and arm wounds.
Kiatikhun 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.
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 centre, 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 that shooting, Anutin, who at the time was interior minister, ordered a raft of gun control measures, including a short-term ban on new licenses, import restrictions, and bans on people under 20 years from shooting ranges.
