Complex sentence: There are two house captains who lead each house in each year, except transition year were there are three captains per house, they act as leaders and one from each year is chosen to represent the year on the student council.
Split into simpler sentences: There are two house captains who lead each house in each year, except transition year in which there are three captains per house. They act as leaders and one from each year is chosen to represent the year on the student council.