Here is a wee snippet I found …
"secondary schools were divided into three-year junior secondary schools, leading to no qualifications, and the five year senior secondary schools, leading to the leaving certificate and university entrance."
I am nor sure if it was standard practice, but I know that some junior secondary schools had a selection process at the end of second year whereby the high performing pupils were transferred to senior high for the remainder of their school life and so could go on to do highers etc
St Gerard's was a senior secondary, but not sure about St George's. If you contact the Archivist at the Mitchell library in Glasgow they have a wealth of info and will be able to tell you. If you have access to the Mitchell they also have registers etc for many schools.
Andrew