A cause of limited physical activity levels in youth is the presence of low back pain (LBP), therefore proper assessment of low back function in physical education settings is needed to identify children who may be at risk. The purpose of this study was to determine the concurrent and criterion-referenced validity of field tests of low back and core muscular endurance in school-aged children. The sample consisted of 4^th through 10^th grade students (N = 370) who completed low back and core muscular fitness tests on four separate testing days during their physical education classes. Field measures related to low back function included the Box 90° Trunk Extension (Box 90°) and the FITNESSGRAM Trunk Extension (FG-TE). Field measures related to overall core function consisted of a Lateral Plank, Prone Plank, and a Static and Dynamic Curl-up. Criterion measures of low-back muscular endurance included the Parallel Roman Chair Dynamic Trunk Extension (PRC-DTE) and the Parallel Roman Chair Static Trunk Extension (PRC-STE). Multivariate analysis using canonical correlations showed moderate correlations between low back and core measures (P < .001). The Lateral Plank, Prone Plank, and Dynamic Curl-up had moderate-to-strong canonical cross-loadings with the low back measure variate. The FG-TE displayed an insignificant canonical coefficient, and weak canonical loadings and cross-loadings. Measures of overall core function also significantly agreed with the criterion measures in classifying students into ranked tertile groups (P < .001). These results suggest that assessment of specific low back muscular function can be easily evaluated using tests of overall core muscular endurance as an alternative to the FG-TE in physical education settings.