Officials at NDG's Villa Maria high school declared a lockdown and started evacuating students on Friday morning after someone sounded an alarm.
There were suggestions that an intruder was present in the school, or on school grounds.
The alarm was pulled at around 10 a.m., and police were called shortly thereafter.
Some students were on lockdown, while others were hustled off to nearby Marianopolis College while police searched the school grounds.
Reports of an intruder were unfounded, and the police operation ended at around 2 p.m.
The police operation ended a few hours later, with no evidence of any emergency happening at the school.