Abstract: This study investigates efficiency differences among high schools in Niğde province of Turkey assuming both constant returns and increasing returns technology in empirical educational production function. The analysis was based on Data Envelopment Analysis (DEA) using linear programming. In addition, to determine whether outliers affects efficiency scores, the Jackknifing method and Spearman rank correlation coefficients were employed. The evidence provided by both of the tests showed that the DEA results are consistent. The results obtained from the DEA analysis imply that all the schools subject to the analysis appear to have had adequate resources, if fully utilized. When the constant (variable) returns technology is employed, the total inefficiency is about 38 (31%) and out of the 35 schools, about 89 (74%) were found to be inefficient. The results that the number of efficient schools has increased when variable returns technology is assumed imply that scale economies are important in education and the use of increasing returns technology in education production function is more appropriate.