The boy, who cannot be named under the Youth Criminal Justice Act, is accused of killing Brett Bourne, also 17, over the lunch hour Tuesday.
An attack against Bourne, a former student, occurred near an entrance to Kelvin High School in the River Heights neighbourhood.
Police have said Bourne was in a minor disagreement with a 16-year-old student.
The 16-year-old backed off and another student intervened and stabbed Bourne.
Police have said the dispute may have stemmed from "relationship issues" — possibly involving a girl — but they have not gone into detail.
There are other unanswered questions, including whether the accused had brought a knife into the school and, if so, for how long.
"The 17-year-old who has been charged obviously had possession. Where that knife came from, where it was taken to, or where it had been, I can't get into those details and I don't have that information at this point," Const. Jason Michalyshen said Thursday.
"That is very likely some information that we're still sorting through."