This study develops a comprehensive theoretical framework regarding the evolution of the class divide in postsecondary education. I conceptualize three prototypes of class inequality--effectively maintained, declining, and expanding--and associate their emergence with the level of competition in college admissions. I also unearth the twin mechanisms, exclusion and adaptation, that link class hierarchy to a highly stratified postsecondary system in an allegedly meritocratic environment. Intra- and inter-cohort comparisons reveal that while the class divide regarding enrollment and access to selective postsecondary schooling is ubiquitous, it declines when competition for slots in higher education is low and expands during periods of high competition. In such a regime of effectively expanding inequality (EEI), a greater emphasis on a certain selection criterion (like test scores) in admission decisions--required to sort the influx of applicants--is bolstered by class-based polarization vis-a-vis this particular criterion. This vicious cycle of exclusion and adaptation intensifies and expedites the escalation of class inequality. The results show that adaptation is more effective than exclusion in expanding class inequality in U.S. higher education. (Contains 18 footnotes, 6 tables, and 3 figures.)