Schools are an attractive setting for implementation of mindfulness-based programs because mindfulness practices, by their very nature, align with a wide range of core educational goals. The present study investigated the effects of an 8-week (16 session) school-based mindfulness program for young children across 8 classrooms (K through 2) using a quasi-experimental delayed-intervention control group design. Results indicated that the mindfulness program was associated with significant improvements in teacher ratings of externalizing and prosocial behaviors. Program outcomes were not associated with child sex or race/ethnicity, but did vary by grade. Descriptive analyses suggest that outcomes tended to be more positive in classrooms with higher levels of teacher and student engagement. Results of the present study add to the growing knowledge base on the positive effects of school-based mindfulness programs and point to a need for more rigorous inquiry into the extent to which students and teachers are engaged with mindfulness programs both during the program itself and in their day to day functioning.