Sarnia police have arrested and charged two youths after a disturbing message was found at Northern Collegiate on Wednesday.
An investigation by detectives identified the teens, who cannot be identified under the Youth Criminal Justice Act.
They were each charged with mischief and uttering threats.
In the interest of safety, the school was closed on Thursday with staff and students moving to remote learning.
Police believe this was an isolated incident and there is no ongoing threat to staff and students at the school.
The Lambton Kent District School Board said classes at Northern will resume in-person on Monday following a PD Day on Friday.
A similar incident was reported at LCCVI in Petrolia last week, which led to the arrest of a 16-year-old from Plympton-Wyoming.