Any faculty member who has been employed in any district for a period of 3 consecutive school years shall enter upon tenure unless dismissed as hereinafter provided. However, a board may at its option extend such period for one additional school year by giving the faculty member notice not later than 60 days before the end of the school year or term during the school year or term immediately preceding the school year or term in which tenure would otherwise be conferred. Such notice must state the corrective actions which the faculty member should take to satisfactorily complete service requirements for tenure. The specific reasons for the one-year extension shall be confidential but shall be issued to the teacher upon request. The foregoing provision for a three-year period and optional one-year extension shall not be construed to interfere with or abrogate local board rules or contracts which now or hereafter may provide for a lesser period of service before entering upon tenure. A tenured faculty member shall have a vested contract right in continued employment as a faculty member subject to termination only upon occurrence of one or more of the following:
     a. Just cause for dismissal; or
     b. A reduction in the number of faculty members employed by the board or a discontinuance of some particular type of teaching service or program.