A gravity-based accessibility metric, the Two-Step Floating Catchment Area (2SFCA) enhanced with gaussian smoothing, is proposed to clarify the distribution of opportunities for compulsory education. Along with the physical locations of the homes and schools, the metric considers the population age structure and capacity to measure the population's accessibility to school services. The application of the 2SFCA within the '15-minute city' model and the analysis of València's urban environment revealed two main findings. First, the distribution of the accessibility scores after a spatial autocorrelation analysis closely reflected the per capita income across the city, irrespective of the type of school and level of compulsory education considered. Second, for most school age children, accessing a charter school is easier than access to a public school for primary and secondary education. The 2SFCA metric appears to adequately capture the uneven spatial distribution of educational opportunities, thus offering new insights into some of the forces the drive school segregation processes and their territorial foundations.