The term “administrator” as used in this chapter applies only to full time, certified professionals having the authority in the interest of a local or regional school district or school within a district to direct, supervise, advise, or manage; provided, however, the term “administrator” does not include superintendents of schools or assistant superintendents of schools. An “administrator” within a teacher bargaining unit, protected by the terms of a regional or local school committee teacher collective bargaining agreement, shall not be protected by the provisions of this chapter.