The paper shows costs of elementary and secondary schools applicable to Oklahoma rural areas, including the long-run average cost curve which indicates the minimum per student cost for educating various numbers of students and the application of the cost curves determining the optimum school district size. In a stratified sample, the school district population was divided into five subpopulations by geographic location and district size. Each region was then stratified into three school district sizes. The sample was designed to be proportional, sampling 5% of the districts in each stratum. The March 1970 survey included 27 independent districts involving 2,225 4th graders, 1,993 8th graders, and 1,903 11th graders. After the schools were randomly drawn, students in the 11th grade of the high schools and students in the 4th and 8th grades of schools that "feed" into these high schools completed standardized achievement tests. Data were also obtained from school administrators. Cost curves are given for: administration, school plant operation and maintenance, instructional activities, buildings, and equipment. The report concludes that the optimum size of a school district is defined as that which has minimum long-run average costs, and that the prescription for optimal economic operations calls for the district to combine resources in a least-cost manner to operate on the long-run average cost curve. (KM)