St Mary's Grammar School (Irish: Scoil Mhuire Machaire Fíolta) is a grammar school in Magherafelt, County Londonderry, Northern Ireland. St. Mary's was first found in 1927. Ten girls were enrolled in the first year under the principalship of Sister Cassian Tracey. The school flourished and grew throughout the 40s under the principalship of Sister Marie Therese Harte. With the school's continued academic success, the need for more accommodation became urgent and in 1954 the first extension was built.