From The New York Times:
The boy brandished a handgun on his Instagram account around Songkran, the Thai New Year, which is celebrated every April.
On Friday, the Thai authorities said, he used the same weapon, which belonged to his grandfather, to kill seven people and inflict fatal injuries on himself. The mass shooting immediately revived a debate about gun laws in the country. The boy shot and killed his grandfather and grandmother, with whom he lived, on Friday morning. Then he went to his high school, the Debsirin Nonthaburi School, northwest of Bangkok, and at some point opened fire.