Toronto police say they were to a scene in the Bayview Avenue and Eglinton Avenue East area at 11:50 a.m.
Police say the 17-year-old victim was stabbed after coming out of his school.
His friend called police and he has been taken to hospital, police say.
Paramedics say the victim appears to have a single stab wound on the left side of his back.
While police say they were told the stabbing occurred during a robbery, investigators have not confirmed this.
Police had initially reported that a suspect had been seen heading westbound along Eglinton Avenue. But police now say they believe the young person they have in custody is the suspect.