Any person employed by a district in a position requiring certification qualifications who serves less than the minimum schoolday as defined in Sections 46112 to 46116, inclusive, or 46141 may specifically contract to serve as a part-time employee. In fixing the compensation of part-time employees, governing boards shall provide an amount which bears the same ratio to the amount provided full-time employees as the time actually served by such part-time employees bears to the time actually served by full-time employees of the same grade or assignment. This section shall not apply to any person classified as a temporary employee under Sections 44919 and 44888, or any person employed as a part-time employee above and beyond his employment as a full-time employee in the same school district.