St. Mary's original school opened in 1917 with 125 students, and by 1958 an addition was built. The school split into two campuses in the 1990's with the original site for grades 4-8, and the new primary grade center for pre-K-3.
The Primary Grade Center, located on the west side of Lake Forest, offers half-day programs for 3- and 4-year-olds and full-day kindergarten. The Upper Grade Center, located on the east side of Lake Forest, underwent a $3.5 million renovation.