There are 36 classes with approximately 1,344 students. The class structure is as follows:
Secondary
1
2
3
4
5
6
7
No. of classes
6
6
6
3 Arts
3 Science
2 Arts
2 Mixed Arts & Science
2 Science
2 Arts
1 Science
2 Arts
1 Science
While the medium of instruction in St. Mary's is English, students are trained to be bi-literate (Chinese and English) and tri-lingual (Cantonese, English and Putonghua).