How is ‘Shakespeare wicked and the map a bad example’ for the children of the school in a slum?

Here, in this line, the poet means to say that just as Shakespeare and his work are of no use to the children in slum school, maps too do not depict the world the slum children can relate to i.e., “narrow streets…. far far from rivers, capes…”. Both Shakespeare and maps represent a beautiful world and high values, which the slum children have never experienced, which could tempt them to steal.