Paramedics responded to Allan A. Martin Senior Public School around 10:45 a.m. Monday and 28 students and one adult were sent to hospital.
Peel Paramedic Services says those taken to hospital had minor symptoms and were taken as a precaution.
Officials from Peel District School Board say it was nothing serious, and school resumed before noon after fire crews aired out the building.
Officials say a Bunsen burner tap that had been left open was the source of the leak.
School officials shut it down before fire crews arrived.