Executive function (EF) skills are correlated with school readiness and academic achievement (i.e. Blair & Razza, 2007; Best, Miller, Naglieri, 2011; Lan, Legare, Ponitz, Li, & Morrison, 2011); however, less is known about the contributions of schooling to EF development. Limited previous research has suggested that some EF skills are sensitive to schooling experiences in the transition from pre-kindergarten to kindergarten (Burrage, Ponitz, McCready, Shah, Sims, Jewkes, & Morrison, 2008). The present study investigated the effects of age and schooling on one type of EF, conflict processing, during kindergarten and first grade. Using a modified child-friendly version of the Attention Network Test (Rueda, Fan, McCandliss, Halparin, Gruber, Lercari, & Posner, 2004), results indicated that conflict processing is significantly more efficient in first graders than kindergarteners with kindergarteners showing a greater reaction time decrement between incongruent and congruent trial times. A school cutoff design was employed in Study 2 to isolate the effect of schooling from maturation on our measures. In contrast to Study 1, developmental change was observed in accuracy scores rather than reaction times. Taken together, these studies capture a developmental shift in conflict processing and provide some insight into how these changes are related to maturational versus schooling effects.