An Education Department official is looking into a school set up by the Mission de l'Esprit-Saint, a religious movement with several branches.
The group had previously operated a school without a permit in Montreal North, but agreed to close it after the department asked it to conform to the rules or shut down.
The department said a visit to the site by its officials in 2004 confirmed it was closed.
However, it appears the school is operating again, the department said, citing a Radio-Canada report. Legal action will be taken if the group continues to break the law, Education Minister Jean-Marc Fournier warned.
The religious movement dates to the 1920s. It split into factions in the 1970s. By then, the Mission had built an underground bunker at Oka and stocked it with millions of diapers and food.
The Education Department also is juggling a separate case involving the religious movement in the Joliette area.
It reached an agreement last year with the group and a local school commission that allowed parents to home school about 140 students. The students' poor test results on department exams have since come to light. The one-year deal required the three sides to meet on Tuesday in court in Joliette. A judge gave them until Oct. 24 to come to an agreement.