A prefabricated building system for constructing a building structure. The prefabricated building system includes a plurality of panels each comprising an outer shell substantially surrounding an inner core. The outer shell comprises a concrete material. The inner core comprises a mixture of a concrete material and particulate foamed polystyrene. A pair of reinforcing meshes are embedded in the outer core so that the inner core is interposed between the reinforcing meshes. Each reinforcing mesh has a plurality of wires arranged in a grid-like fashion forming a plurality of generally square spaces therebetween.