Huronia West OPP say Elmvale District High School was vandalized.
The incident happened around 12:30 a.m. on March 23.
Police went to the school in response to an alarm that was triggered.
"Upon arrival it was discovered that unknown individuals used a 20-pound propane tank to break the south window of the athletics department," Const. Mark Kinney said. "Fortunately the school did not lose any property in this crime."
Anyone with information about the incident is asked to call the detachment at 429-3575.
Tips can also be left with Crime Stoppers at 1-800-222-TIPS.