Objective: The aim of this 18-month longitudinal study was to explore the psychometric properties of the recently developed Working Memory Rating Scale (WMRS) within a general school population of 524 six- to nine-year-old children (259 boys, 265 girls) and with an examination of sex and time differences. Method: Teachers completed the WMRS and children completed objective measures of WM and standardized measures of academic achievement. Results: Confirmatory factor analyses indicated a poor fit for the 20 original WMRS items. Post hoc analyses, however, revealed that the factor structure of an alternative five-item short form was strong for both boys and girls and at the two time points, spanning two consecutive academic years. Internal consistency, criterion-related validity, and convergent validity of this alternative five-item WMRS were also excellent. Conclusion: The short five-item WMRS may eventually provide teachers with a useful and time-effective method to screen for WM deficits at school.