The purpose of this study was to analyze student academic performance data from year-round calendar high schools across the United States in comparison to those of traditional calendar high schools within the same states. This study sought to determine if the mean passing scores of students for the last three academic years in four important sub-groups of total school population, students who receive special services, English Language Learners, and children from low-socio-economic backgrounds, were significantly different from the mean passing scores of students from the same sub-groups who attend schools with traditional calendars. The student and school data used for this study were collected from only documented public, non-charter, high schools that operated on a 12-month, year-round calendar in the United States during the years of 2007-2010. These 26 high schools in the three states of California, Illinois and Texas were then similarly matched schools with 26 schools from traditional, 10 month calendar high schools. This study implemented a Causal-Comparative Design using Independent Samples t-tests to compare the twenty-six year-round schools to the twenty-six traditional calendar schools. The results of this study showed no statistical significance regarding the p-values of each subgroup from each state for math and language arts. These results also revealed that across all three states, and in all four sub-groups, traditional-calendar high schools consistently outperformed their year-round peers in math and language arts from the academic years of 2007 to 2010. The following are appended: (1) California Traditional Calendar High Schools; (2) California Year-Round Calendar High Schools; (3) Illinois Traditional Calendar High Schools; (5) Illinois Year-Round Calendar High Schools; and (6) Texas Traditional Calendar High Schools. (Contains 2 tables and 3 charts.)