Complex sentence: The headmaster, his assistants and the senior housemaster/housemistress are in charge of the day-to-day administration of the school, assisted by the house staff in charge of the boarding system.
Split into simpler sentences: The headmaster, their assistants, and the senior housemaster/housemistress are in charge of the day-to-day administration of the school. They are assisted by the house staff in charge of the boarding system.