The hypothesis of this study was that student course-taking on the block schedules contributes to the comprehensive nature of the high school and the differentiated curriculum. Because of the shift of time associated with the block schedule, it was hypothesized that students would spend significantly less time studying the core academic curriculum, and more time studying noncore subjects. The study also hypothesizes the amount of time a student spends studying the academic core curriculum is a predictor of success on a test of postsecondary readiness. A proportional stratified random sample of 288 transcripts was selected from the graduating classes of one high school with a recently adopted alternating day A/B block schedule and one high school using a 4x4 semester block schedule. The results call into question the importance of the delivery of curriculum evolving with the implementation of block scheduling. In these schools, the block schedule has created a time shift, rather than a credit shift, away from core academic subjects to elective areas. The regression indicates that this time shift can be harmful to student achievement since the hours spent studying the academic core proved to be a strong predictor of success on tests like the ACT Assessment. While the raw achievement data are mixed, a general decline in test scores is evident in mathematics, especially among students studying a rigorous precollege curriculum. Schools that embrace block scheduling to increase achievement by adjusting the learning time are using the block schedule as it was intended to be used, but those that are using block scheduling to give students increased opportunities to take electives are putting aside a rigorous academic curriculum. Four appendixes contain graduation requirements for both schools, school day schedules, ACT scores for both schools, and regression analysis tables. (Contains 6 tables and 53 references.) (SLD)