Teachers who are required, because of additional work, to devote more than the usual number of periods per day to their duties, shall be entitled to a fair increase in compensation to be determined by the board of public education or the board of school directors. Teachers, who may be employed in giving instruction for only part of a day, shall render such other service for such period of time per day as the board of school directors may direct, but if such service cannot be assigned to such teacher by the board of school directors, the salary paid to such teacher shall be proportioned to the number of hours of service rendered.