The Education Department has thrown its support behind a school that invited students to leave assembly during the singing of the national anthem.

Cranbourne Carlisle Primary School recently told Shiite Muslim students that they could excuse themselves from singing Advance Australia Fair because it was a religious month of mourning.

The move infuriated Lorraine McCurdy, who has grandchildren at the school and told 3AW radio that up to 40 students left the room.

"Two children got up and said 'welcome to our assembly'. With that, a teacher came forward and said all those who feel it's against their culture may leave the room," she said.