Peel Region police say two more students — 16- and 17-year-old boys — were arrested at the time.
Police said on Tuesday evening that another 16-year-old boy who was being sought was also in custody.
Const. Fiona Thivierge says police were called to St. Roch Catholic Secondary School in Brampton, Ont., just before 11:30 a.m. and promptly placed it under lockdown.
The lockdown ended after police determined the third suspect is not in the school.
And hold and secures were ended at other area schools.