Sarnia police responded to Great Lakes Secondary School (GLSS) Friday morning.
The Lambton Kent District School Board (LKDSB) said it was made aware of an alleged threat directed at the Murphy Road school.
Police were notified and following a thorough investigation of the grounds, it was determined the school was safe to open and they're not concerned the threat was a legitimate one.
"Communication was sent to all staff and school families prior to the start of the school day to keep them informed of the situation," read a statement from the school board.
The statement said as an added precaution, Sarnia police and the LKDSB safe schools coordinator will remain on site for the duration of the school day.
"The safety and wellbeing of our students, staff and school community is our highest priority. The LKDSB will continue to work closely with Sarnia Police Services to ensure a safe and supportive learning environment for all," the statement added.