Jimmy said pupils were locked in their classrooms for two to three hours.
“[When police cars arrived] I thought this is getting serious, I’m getting worried, I don’t know what’s going on,” he said.
The 15-year-old said he was able to message his mother when the school let students use their phones after about an hour.
“When I was able to eventually get out, she was giving me a bunch of hugs,” he said.
“What I am is shocked. I’m surprised that this happened,” he added.
Jimmy said the school had previously run lockdown drills but a real event had never happened before.
Adam, whose daughter attends the same school, said he felt frightened even though he had texted her and knew she was OK.
“I don’t think this should happen at school. Thank God she is safe,” he said.
The school had messaged parents to say everything was under control, he added.