During the school semester, students face an onslaught of new material. Students work hard to achieve initial mastery of the material, but soon their skill degrades or they forget. Although students and educators both appreciate that review can help stabilize learning, time constraints result in a trade off between acquiring new knowledge and preserving old knowledge. To use time efficiently, when should review take place? Experimental studies have shown benefits to long-term retention with spaced study, but little practical advice is available to students and educators about the optimal spacing of study. The dearth of advice is due to the challenge of conducting experimental studies of learning in educational settings where material is introduced in blocks over the time frame of a semester. In this paper, we turn to two established models of memory---ACT-R and MCM---to conduct simulation studies exploring the impact of study schedule on long-term retention. Based on the premise of fixed time each week to review, converging evidence from the two models suggests that an optimal review schedule obtains significant benefits over haphazard (suboptimal) review schedules. Further, we identify two scheduling heuristics that obtain near optimal review performance: (1) review the material from u-weeks back, and (2) review material whose predicted memory strength is closest to theta. The former has implications for classroom instruction and the latter for the design of electronic tutors.