This paper investigates urban-rural and size effects on public upper secondary school efficiency in Northern Thailand. Efficiency is assessed by a nonparametric technique, namely data envelopment analysis. Urban-rural and size effects are examined through a Mann-Whitney nonparametric statistical test and by Tobit regression analysis, respectively. Results indicate that rural schools operate less efficiently than urban ones. School size contributes positively to both types of schools, while class size has a positive effect on urban schools and a negative one on rural schools. Policy to improve school efficiency should thus focus on rural schools, expanding school size while reducing class size. (Contains 3 tables and 2 figures.)