Police say they received a tip through Crime Stoppers about the presence of a gun at Blessed Mother Teresa Catholic Secondary School at 40 Sewells Rd., near McLevin Avenue and Neilson Road.
The school was subsequently put into lockdown, as officers looked for the weapon.
Police allege that the gun had been placed in a bag that "was in the possession" of four young people, whom the school board has confirmed were students.
It is further alleged that one of the accused, a young woman, tried to leave the school during the lockdown. But she was arrested and police found the gun in the bag.
All four of the young people have been charged with 10 firearm-related criminal counts, including carrying a concealed weapon, careless use of a firearm and careless storage of ammunition.
The Youth Criminal Justice Act prevents the accused from being publicly identified.
The four accused were due to appear in a Toronto court on Thursday morning.