The incident happened at the Catholic High School of Notre Dame in 1375 Harwood Ave. N., just north of Rossland Road.
Durham Regional Police said officers were at the scene at about 2:35 p.m. after reports that the school had received a “threat of violence”.
No injuries were reported.
As a precautionary measure, Notre Dame de Paris and J. Clarke Richardson College, whose building is adjacent to this school, were locked down. The lockdown was lifted shortly after 3:30 p.m.
Police said their investigation identified the “source of the threat and a student was arrested” shortly afterwards.
The accused, whose identity can not be revealed under the provisions of the Law on Criminal Justice for young people, was released on bail.
Durham Police reminds the public that threats against schools are taken “very seriously” and urges anyone with information about the incident to contact them at 1-888-579-1520, etc. 3962 or Crime Stoppers anonymously at 1-800-222-8477 (TIPS) or online at www.durhamregionalcrimestoppers.ca.