Using data from the Early Childhood Longitudinal Study Kindergarten Class of 2010-2011 (ECLS-K: 2011; n = 11,000), this study examined the developmental outcomes of 5-year-old children in multi-grade classrooms (combined pre-kindergarten and kindergarten classrooms serving 3-, 4-, and 5-year-olds) compared with 5-year-olds attending kindergarten-only classrooms serving primarily 5-year-olds. Results from regression and propensity score analyses revealed that 5-year-old children who attended multi-grade classrooms with pre-kindergarteners made smaller gains in math and literacy skills and demonstrated less optimal executive function at the end of the school year as compared with children who attended kindergarten-only classrooms. Classroom-level factors largely explained the differences in children's academic achievement but did not consistently explain differences in their executive functioning. No consistent differences emerged for children's social-behavioral development. [This paper was published in "Infant and Child Development" (EJ1162658).]