Police responded to Woburn Collegiate Institute on Ellesmere Road, just east of Markham Road, around 3:20 p.m.  on Monday.
According to police, the victim was shot in front of the school.
Emergency crews arrived and rushed the boy to hospital where he was later pronounced dead.
Toronto police identified him Tuesday as 18-year-old Jefferson Peter Shardeley Guerrier of Toronto.
After the shooting, a 15-year-old boy went to a nearby hospital with gunshot wounds.  He was taken to another hospital where he is now in stable condition.
Investigators described the shooting suspect as a teenage boy with a thin build who wore dark clothing.
It was not immediately clear if any of the victims attended the school.
Speaking outside the school Tuesday morning, TDSB Director of Education Colleen Russell-Rawlins told reporters that support staff are on scene this morning to speak with teachers and students and will remain available as long as needed.
“We’re all here to listen to their concerns and answer their questions very directly,” Russell-Rawlins said.  “We also have extra adults in the building today.  But they know they can count on the people they see every day to support them.
“Whether it’s school security or vice principals, our kids and youth, their teachers — they’re all here because we know that when students are in trouble, when they need help, the people they go to are in this building every day .  And so we can prevent issues like we’ve seen here.”
He called the killing on school grounds “a horrific tragedy” and said the board would “consider very carefully what needs to be done here and in our schools to continue to provide safety and support for students” following the police investigation.
Russell-Rollins said a “multi-pronged” approach is needed to prevent violent crime at school, including action by the school board and different levels of government.
The fatal shooting occurred near the end of the school day Monday afternoon and put the school on lockdown.  Police began escorting students out of the building around 4:30 p.m
Police are asking anyone with more information to contact investigators or call Crime Stoppers anonymously.