Earlier Tuesday, Peel Regional Police Const. Fiona Thivierge reported that two students — aged 16 and 17, both boys — had been arrested, while another 16-year-old boy was being sought.
"It was an altercation between two males in the hallway of the school that escalated with other parties coming into the altercation, which resulted in the stab wound," Thivierge said.
A 17-year-old male student was taken to hospital with non-life threatening injuries.
Const. Thomas Ruttan later told CBC News that the third suspect had been arrested.
Thivierge said police were called to St. Roch Catholic Secondary School in Brampton, just before 11:30 a.m. ET and promptly placed it under lockdown.
The lockdown ended after police determined the third suspect was not in the school.