A high school student in Zama, Kanagawa Prefecture just south of Tokyo sprayed what he alleged to be pesticide at school on June 30.
According to police, a teacher reported that a student sprayed tear gas on schoolgrounds just before 11AM.
The student sprayed the gas on the third-floor hallway on the way to the next class.
18 students and teachers reportedly had a hard time breathing and 13 of them were sent to the hospital and are in stable condition.
The boy who allegedly sprayed the gas is in custody.
He said that he sprayed pesticide as a bad prank and threw the pesticide out of the window.
Police are investigating the contents of the spray as they seek further detail.