School anxiety is one of the most prevalent school attendance problems in Spanish children. Despite its importance, no instruments are available for a precise diagnosis and to establish gender and age differences. This study analyzed the measurement invariance and the latent mean differences of the School Anxiety Inventory for Primary Education (SAI-PE) scores across gender and age in a sample of Spanish children. The sample comprised 1,203 children between 8 and 11 years (M = 10.2, SD = 1.32), selected through random sampling. Findings showed that the SAI-PE presents measurement invariance across gender and age in Spanish children. The latent means analysis revealed that girls scored significantly higher than boys on all SAI-PE dimensions. School anxiety was significantly higher among eight-year-old students. The SAI-PE is the only instrument reliably assessing school situations and anxiety responses in the Spanish child population.