It happened around 3:05 p.m.  at Birchmount Park Collegiate Institute near Danforth Avenue and Birchmount Road.
Police say they located a 17-year-old male in the school office suffering from an apparent stab wound.  Police “immediately began lifesaving measures” before paramedics arrived and transported the victim to hospital, where he remains in life-threatening condition, police said.
Toronto District School Board (TDSB) community relations officer Ryan Bird confirmed the victim is a student at Birchmount Park.
Police are currently clearing the school and assisting in the safe dismissal of students.
Police say the investigation is ongoing, however they do not believe there is a concern for public safety.
No suspect information has been provided.
“We have officers going from room to room to make sure it’s safe.  At the same time we also have officers assigned to assist in the safe dismissal of students.  They are dismissed on the east side of the school at the entrance [number] four.  The parents have been told to settle there, where they can be reunited with their families at that time,” said Toronto duty officer Craig Young at the scene.
“At this time we have appointed detectives.  Several detectives are here and elsewhere, following the leads of this case.’
Bird said support workers are currently at the school to assist students and staff.
“Social workers are on site this afternoon to support everyone affected by what happened here this afternoon.  Obviously, our hearts go out to the victim and we hope he makes a speedy recovery,” Bird added.
The high school was initially placed on lockdown after the stabbing, which has since been lifted.
It is the second stabbing at the school so far this year.  In April, a 14-year-old boy was stabbed outside Birchmount Park Collegiate shortly after students dismissed for the day.  The teenager was seriously injured in the incident. 
The stabbing also comes two weeks after a shooting outside Woburn Collegiate Institute, another high school on the city’s east end, left one student dead and another injured.
“Schools deserve to be a safe place to learn and work,” Bird said.
“While the TDSB plays an important role in this solution, I think we need help from other levels of government and community partners. This is not something the TDSB can do alone,” he said.
After being dismissed from his classroom after two and a half hours on lockdown, Kkid Kkid, a Grade 12 student at Birchmount Park Collegiate, said students were given little information and told to stay put until police arrived.
“It’s a great school. Things like that happen, but there are great teachers, great staff,” he said.  “Hopefully these events don’t allow people to just categorize the school as a bad place.” 
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With files from The Canadian Press