The suspected shooter, a high school student, was later found dead, bringing the total number of fatalities to seven at Debsirin Nonthaburi School in Bang Kruai district in Nonthaburi, Bangkok Post reported.
The incident was reported at around 10 am when volunteers from the Poh Teck Tung Foundation received reports of shots fired inside the well-known secondary school. A student inside the school said the gunman, dressed in a purple physical-education uniform, fired dozens of rounds on the school grounds.
According to reports, the fatalities include three teachers, three students and the suspected shooter. Fifteen injured people were taken to hospitals for treatment.
The suspected perpetrator, reportedly a Mathayom 3 (Grade 9) student, had barricaded himself on the third floor of the school's pink building. Police confirmed the suspected gunman was found dead, apparently of self-inflicted wounds.
Authorities rushed to the scene to evacuate students and staff and maintain a security cordon around the campus.
Officials urged the public to stop livestreaming the incident and avoid the surrounding area to prevent interference with the ongoing operation.