Police blocked off nearby intersections and locked-down the school for more than an hour, barricading students in their classrooms.
Anxious parents rushed to the school after getting text messages from their children. They were forced to wait outside, barred from the school by police.
Longueuil police constable Mark David said authorities found an air gun in a boy's locker and arrested three 16-year-old students.
Once the coast was clear, students were released from their classrooms shortly after noon, spilling out of the school and into the arms of their relieved parents.
"It was really stressing," said Rosaliève Beaulé, 16. "It's been a really tough time."
Francis Therrien was in one of the classrooms where police arrested a classmate.
"Around 11:30, police came in. They blocked the door. They apprehended a suspect," he said, adding every student was searched before being allowed to leave the classroom.
Police would not say much about the arrests, as the three facing charges are all minors. School officials said none of the boys was a problem student.
Classes were cancelled for the remainder of the day Monday and will resume Tuesday. School officials said psychologists will be on hand to talk to students and staff who want counselling.