Despite increasing interest in improving academic outcomes for students by enhancing mindfulness, there is a paucity of evidence that greater mindfulness is associated with success in school. We measured mindfulness with the short-form Mindful Attention Awareness Scale (MAAS) in over 2,000 urban students in Grades 5-8. The MAAS had good internal consistency and scale homogeneity. Greater mindfulness correlated significantly with better academic achievement as measured by grade point average and standardized tests of mathematics and literacy, greater improvement in academic performance from the prior school year, better attendance, and fewer suspensions. The relation between mindfulness and academic achievement was similar across demographic characteristics. These findings support the reliability of the MAAS as a measure of mindfulness among youth and provide initial evidence of an association between mindfulness and academic achievement. This association strengthens the rationale to explore whether mindfulness-based interventions can enhance academic outcomes by leveraging the malleability of mindfulness.