Peel Regional Police were called to a shooting in the area of ​​Gore Road and Fitzpatrick Drive shortly after noon.
The shooting happened outside a high school parking lot and is believed to have been targeted, police said.
An 18-year-old student from the school is now in hospital with life-threatening injuries, police said.
In a statement, the Peel District School Board confirmed that the shooting occurred outside Castlebrooke Secondary School.
“Police were called to the scene and are investigating.  The student has been taken to the hospital.  At this time, we have no other information to share,” the school board said in a statement.  “Castlebrooke Secondary School staff and school board staff are working closely with Peel Regional Police on the investigation.”
The council said it is currently providing support to students and staff at Castlebrooke Secondary.
“The safety, well-being and mental health of our students is our first priority,” the board said.
One suspect is currently outstanding.  There is no suspect description so far.
Police said they do not believe there is a risk to public safety.