As a condition for receiving funds under this article, it shall be the duty of each local unit of administration to employ and to utilize the services of substitutes for teachers who are absent. It shall be the duty of the local unit of administration to employ substitutes, including retired teachers in accordance with Code Section 47-3-127, who possess valid teaching certificates issued by the Professional Standards Commission. If no person holding a valid teaching certificate is available for this purpose, the local unit of administration is authorized to employ the person who most closely meets the requirements for certification as a teacher and who is available to serve as a substitute, provided such person is closely supervised by the school principal or principal's designee. It shall be the duty of the commission to promulgate and adopt rules, regulations, and policies establishing classes or categories of persons, in order of descending priority, who most closely meet requirements for certification within this state. Nothing contained in this Code section shall prevent the local governing board or its executive officer from refusing to employ as a substitute teacher one who, in the discretion of the board, would be detrimental to the education of the students provided for by this article. For purposes of this Code section, certificated substitute teachers shall not be considered part-time personnel pursuant to Code Section 20-2-212. Salaries for substitute teachers shall be set by the local boards of education.