This study examines the associations between class size, teacher characteristics, and children's academic achievement in mathematics, reading, and science using a recent, large-scale, longitudinal dataset. Individual fixed effects models, which control for observed and unobserved time-invariant student variables, were employed to conduct analysis of longitudinal data from the Early Childhood Longitudinal Study -- Kindergarten in 2011 (ECLS-K:2011). Results suggest that reducing class size is associated with increases in reading and mathematics scores, but not in science scores. Non-linear class size effects were also detected and were more pronounced in reading achievement. Class size estimates were small in magnitude. A five-student decrease in class size corresponds to nearly a 0.01 standard deviation increase in mathematics and reading scores. None of the teacher characteristics were related to children's cognitive outcomes. Finally, changing schools from grade to grade has a significant negative effect on children's reading achievement.