Halifax Regional Police were called to Citadel High School on Tuesday after someone sprayed a sensory irritant inside the building.
Const. Pierre Bourdages told CBC News the students were kept outside the school on Trollope Street while the building was ventilated during an extended lunch break.
Bourdages said the investigation is in its early stages and did not say whether a student was targeted with the irritant. He said he did not have details on what the sensory irritant was.
Classes at Citadel High School had resumed as of 1 p.m.