No assessor shall be employed who has not been licensed as qualified by the board, provided the time to comply may be extended after application to the board upon a showing that licensed assessors are not available for employment. The board may license a county or local assessor who has not received the training, but possesses the necessary qualifications for performing the functions of the office by the passage of an approved examination or may waive the examination if such person has demonstrated competence in performing the functions of the office for a period of time the board deems reasonable.