The plan has been revised to account for the predicted future growth of the town.
A major new building is being planned to house the Marist College in Athlone.
School authorities will consult with the Department of Education to plan the scale of the development including new classrooms, auxiliary rooms and specialist facilities.
The school was originally approved for refurbishment and extension but this was revised to account for the predicted future growth of the town.
Junior Enterprise Minister and Longford-Westmeath Fianna Fáil TD Robert Troy is welcoming the announcement: