It is an opportunity to celebrate their five years at school, and to recognise in a formal way their transition to their next phase of education. They will not be together as a group again as they move on to sixth-form college, further education, vocational training or employment.
Year 11 spends the month beforehand developing the liturgy, selecting the hymns and readings, and texts for the post-communion mediation. The head boy and girl write and practise their speeches. The musicians rehearse appropriate set-pieces.
Then there is the added special ingredient. This year it was a 10-minute PowerPoint presentation of photographs of Year 11's time at the school. The pictures were artfully edited, with wittily apposite captions. The young lady who prepared the presentation has a bright future as a photo-journalist or editor of reality TV shows.
Last year the highlight was the rock band, which had been enlivening religious festivities for some time, and playing regular secular gigs in Cardiff. One year there was a brilliantly scripted and performed chat show, which expanded on the Gospel readings with an imaginative contemporary slant. The chat-show host is now a professional actress.
These pupils are just 16, but the school has given them a maturity and confidence beyond their years. This is why we struggle through finance meetings, manage lean budgets, and tackle painful staff reorganisations, pupil disciplinary panels and inspections.
This is what our contribution is about - enabling young people to face the world with an appropriate set of skills, knowledge and a strong sense of right and wrong.
Many schools in Wales will lose their sixth form in the next few years. We lost ours 15 years ago when what is now St David's Catholic college, in Cardiff, the first sixth-form college in Wales, was created.
Ours is now a better school academically, pastorally and religiously than when it had a sixth form. The range of opportunities available to our pupils is tremendous. Don't believe us? Then believe the inspectors who gave us the once-over last term.
Dr Martin Price is chair of governors at Richard Gwyn Roman Catholic high school, Barry, in the Vale of Glamorgan