I'm thinking it might be The Girl Who Owned a City by O.T. Nelson. It was written in 1975 and during the '80s was often an "enrichment" selection on grade school book lists, meaning it was given to children who breezed through the regular class reading lists.
A virus kills all the adults, and society falls apart. A girl and her younger brother become the central unifying figures of a group of children who establish a society in a school building, which they fortify and barricade against marauding child gangs, so that a fair part of the book has the protagonist-children living exclusively indoors in semidarkness.
