Do students attending school on a year-round calendar outperform students attending school on a traditional calendar in reading and math? Mixed and inconclusive findings are reported in previous studies. This study examined the reading and math achievement of 2004-2005 fifth graders in three school-within-a-school year-round elementary schools located in the United States. An ex post facto comparison group posttest design was utilized. National percentile ranks on state selected standardized tests were analyzed. Overall, sixteen comparisons of year-round and traditional student achievement and growth were made. When mean scores were compared in reading and math achievement and growth, all four comparisons favored year-round education. Only one of these four differences, fifth grade national percentile rank, was statistically significant. When student level variables were controlled, four reading comparisons were not statistically significant. However, all four math comparisons were statistically significant when student level variables were controlled. This study found that year-round calendar students statistically outperform traditional calendar students in a school-within-a-school setting in Mathematics;