Of the six persons to be then elected together out of each of the classes of heads of colleges, or halls, professors, and members of Convocation, the three juniors of each class in academical standing, reckoned from matriculation, shall vacate their seats at the expiration of the third year from such day within the then current academical year as shall be named by the Hebdomadal Council in that behalf; and all the other persons to be then elected shall vacate their seats at the expiration of the sixth year from the said day; and all other persons elected from time to time, except such as shall be so elected upon casual vacancies, shall vacate their seats at the expiration of six years; and the election to supply the places of the persons so vacating their seats shall be made upon the day on which seats are vacated.