The school in Whitby, which was not identified, was put on hold-and-secure mode Thursday while police investigated a photo posted on Instagram.
Durham regional police say the photo, which depicted the student holding a weapon, was more than a year old and was taken outside Canada.
They say there was never an immediate threat to students, but protocols were followed to ensure the safety of the school.
The student co-operated throughout the investigation and officers confirmed he did not own or have access to a weapon.
Police are warning the public that irresponsible use of social media can lead to criminal investigations when images or comments depict any criminal activity.