Before closing, St. Mary's had shifting sports rivalries. For at least 20 years, St. Mary's main sports rival was St. Pius X High School; that rivalry ended when St. Mary's expanded its enrollment. Another rival was Grain Valley High School; it ended when St. Mary's enrollment fell to 200 students. In 2010, Van Horn High School, joined the Independence School District and became a member of the Crossroads Conference. The two schools first met on the football field in September 2010 for the St. Mary's homecoming football game.