In common with all state-run Jamaican secondary schools, St. Mary High uses a selection process to accept students at the grade seven level.

The Portals of St. Mary High opened on January 12, 1960 at 8:30 a.m. to 69 pupils (31 boys and 38 girls) and four teachers including the principal, Mr. L. W. Brown, B.Sc.

Expansions did not stop, as a World Bank project extended the "B" Block which housed the Home Economics and Business Departments.

The House system was introduced in January 1962; two years after the school first opened its doors and the first inter-house sports competition in March 1962.

Although St. Mary High School is not a boarding institution, foreign or out of town teachers have on-campus living options with apartment style cottages across from the nurses and guidance counselors stations.

The school has a spacious volleyball/netball court that is often called the Cadet Square and a football field across the main roadway that runs parallel to the campus.

Male sixth form students wear the traditional khaki pants along with a white short or long sleeved shirt, school crest attached to front pocket, and the same tie worn by their female counterparts.

Subjects offered to students include Technical Drawing, Building Technology, Industrial Technology, Mathematics, Physical Education, General Science, Social Studies, Home Economics, Biology, Geography, History, Physics, Food and Nutrition, Clothing and Textiles, Principles of Accounting, Office Administration, Principles of Business, English, Sociology, Music, Chemistry, Information Technology, Visual Art, Literature, Law, Spanish, Agricultural Science, and Religious Education.