The school belongs to the French-language school board, Commission scolaire de Montréal.
Inspectors found water was making its way into parts of the roof of the two-storey building, causing mould to develop.
In June, contractors will begin work to fix the problem, but the job could take up to three years.
As a result, the students will have to find a new school for September 2015.
There is a school belonging to the English Montreal School Board next door.
But the EMSB has converted St. Ignatius of Loyola to accommodate students with special needs, and at risk of dropping out.
The nearest French-language schools are several kilometres away.