Nonthaburi provincial police commander Lt Col Dechrapee Kongdee said the suspected gunman had committed suicide.
Among those injured were one teacher and three students, he said.
The incident took place in the Bang Kruai district of Nonthaburi province north of Bangkok. Police have identified the perpetrator as a student.
One 18-year-old student said 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.”
In photos circulated by emergency workers, students streamed out of the Debsirin Nonthaburi School, as ambulances arrived.
In one photo, one person is seen lying down on a stretcher outside an ambulance, while another is attended to by a medic.
In the 2025 academic year, the school had an enrolment of around 3,100 students and 147 teachers, according to district authorities.
In February a teacher died and a student was injured in southern Thailand’s Hat Yai district after a gunman opened fire at a school. (Reuters)
