(a) A professional educator who is employed at or above seventh grade level to devote a substantial portion of his time to instruction in one or more of the core academic subjects in a grade or grades in which instruction is departmentalized shall be deemed to serve in the academic tenure area or areas encompassing such subjects.

(b) There shall be five academic tenure areas, reflecting the division of the instructional staff of a departmentalized school into separate units for giving of instruction in English, social studies, mathematics, science and world languages.