The school, founded in August 1857 to provide education for boys in the greater Belfast area, held an open afternoon at the end of term as part of the celebrations.
Former pupils were invited to attend and reminisce over photographs and a buffet lunch.
Later in the year there will be a former headmasters and head boys lunch, gala dinner, carol and thanksgiving service and a football match featuring former pupils against staff.
In February it was announced that the school would get a multi-million pound new building with state-of-the-art facilities.
As part of the anniversary celebrations, the school is compiling a film highlighting significant events in the school's history.
Past pupils willing to contribute photographs, video or other relevant material can email firstname.lastname@example.org.
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