Lethbridge police say two young men got into a fight outside Chinook High School during the lunch hour Wednesday.
One of them discharged the substance and as many as 25 students watching the conflict got hit when wind blew it in their direction.
Sgt. Ryan Stef says a 15-year-old suspect, who is not a student at the school, fled the area.
The teen was arrested a short time later by patrol officers.
Stef said the case remains under investigation and charges are pending.