Mindfulness-based practices for school-aged children are increasingly common in the United States. Positive and negative affect are theoretically and empirically associated with school outcomes, and these constructs are likely to be impacted by school-based mindfulness practices. Furthermore, mindful states, such as being calm and focused, targeted by mindfulness-based practices are a potential causal mechanism to improve learning and behavior. This study describes a test of longitudinal factorial invariance for a brief measure of affect states plus a state of calm focus that is appropriate for use in mindfulness intervention studies with elementary school-aged children. Data were collected from 97 fourth-grade students in an urban elementary school that was about 46% Hispanic and 44% White. Students listened to one of six different conditions each day for 24 school days: There were four individual mindfulness modules (cross-connect, pause buttons, belly breathing, and mindful minute), one condition that included all four of the modules together, and one condition that was a control activity (a grade-appropriate story presented via audiorecording). Students provided self-report of positive and negative emotions plus calm focus immediately before and after the presentation of the audio. Results showed high levels of internal consistency for the scales, low correlations between scales, and factorial invariance in the pre- and post-test design for five of the six conditions. These findings support the use of this measure in studies with older elementary school students. Future studies should further document construct validity through external validation of the scales.