Students streamed out of the Debsirin Nonthaburi School, on the northwestern outskirts of Bangkok, as the wounded were loaded onto ambulances and teachers hugged each other and cried.
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 on Friday.
"There were many shots: bang bang bang. Then it went quiet. Then it started again."
Lieutenant Colonel Trirong Phophan, a spokesman for the National Police, told Reuters that six people were killed, along with the suspected shooter.
In an interview with news broadcaster Channel 3, Trairong said three teachers and three students were killed.
Fifteen people were injured, including two in a critical condition.
Police identified the perpetrator as a student.
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.
"We gave CPR for about 30 minutes and rushed her to the hospital as we could not get her heart rate up, but we tried our best," he said.
In the 2025 academic year, the school had an enrolment of about 3100 students and 147 teachers, according to district authorities.
Deputy Education Minister Akkharanan Kannakittinan said in a statement he was co-ordinating to provide assistance.
Gun ownership and gun violence are not uncommon in Thailand, which has seen a series of deadly shootings in the past six years, with children among the victims.
Thailand's worst mass killing by a single perpetrator in its recent history took place in 2022, when a former policeman ran amok in a three-hour gun-and-knife attack in the northeast that killed 36 people including 22 children stabbed as they slept at a daycare centre.
A gunman killed five people including security guards and a vendor at a Bangkok market in July 2025 before turning the gun on himself, while a teacher died in the southern city of Hat Yai in February this year when a shooter opened fire at a school where she worked.
In 2020, a soldier killed 29 people in a shooting rampage in Nakhon Ratchasima.
And in 2023, a 14-year-old used a modified handgun to kill two people and wound five others at a luxury Bangkok shopping centre.
with AP
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