In this paper, academic efficiency is operationally defined and a methodology for measuring it at the school level is described. Academic efficiency is defined as the extent to which a school utilizes its time for the academic development of all its students. The measure of academic efficiency must include three elements: time, students, and academic development. The metric used is the student-minute; one student-minute is a minute spent by one student. Assuming that academic engagement is the legitimate proxy of academic efficiency, the index of academic efficiency is the proportion of engaged student-minutes to total student-minutes. To calculate this index, six different types of data must be collected at the school level: total time available in school day, time scheduled for noninstructional activities, total enrollment, average number of students absent per day, average engagement rate, and average proportion of nonacademic instructional time per class. Procedures have been developed at the Mid-Continent Regional Educational Laboratory for sampling engagement rate and non-academic instructional time. (BW)