The St. Cenydd School Senedd is an assembly of appointed pupil representatives to express their opinions about the school, to voice their opinion on how it can be improved, as well as undertake research and collaborative work through a committee structure to bring about change.

The Senedd consists of pupils from each year group, appointed following an application/interview process. The Head Boy and Head Girl of the school chair during half-termly whole school Senedd sessions.

Between whole school Senedd sessions, Senedd members sit on committees working alongside nominated members of staff, to draft a white paper for consideration and voting/ratification by the whole school Senedd in the next whole Senedd session. The nature and volume of committees are chosen by members of the Senedd in the first whole Senedd meeting.