Mind wandering, where attention drifts from the here-and-now to internal thoughts, is often linked to decreased educational performance. However, its impact on children remains largely unexplored. This study introduces and evaluates a method for measuring mind wandering in classroom environments. A sample of 126 8-9-year-olds participated in a listening activity where they periodically categorized their thoughts before completing immediate and delayed memory tests. Children reported being off-task in 45% of responses, 22% of which were mind wandering and 23% of which were thoughts about distractions. Our findings reveal that classroom mind wandering is common and can adversely affect immediate memory recall. The long-term implications of mind wandering in childhood are still uncertain, highlighting the need for further research. The results demonstrate the feasibility of assessing off-task thought in the classroom using thought probes that differentiate between distractions and mind wandering, which have distinct causes and may require tailored responses.