Virtually every juku classroom that I’ve ever visited has a world map on the wall. You know the one with the flags of all UN members arranged around the outside of the map. These are generally available at the ¥100-shop (equivalent to Dollar Stores).
Generally, these maps are quite decorative in that they are colourful and with their B3 size take up a bit of wall space. What I do find surprising is how universal this is as decoration.
In addition to being decorate, the world map obviously also includes information, namely the names of countries, important geographical features, etc. Much of this information may also show up as test material in social studies classes.
However, world maps are not the only imaginable decoration-cum-learning-content. What about a periodic chart of elements? Or, geometrical shapes and their accompanying formulae for area and volume? Geometry takes up substantially more teaching time and commands more attention (in part because arithmetic/mathematics is a subject that features on more entrance examinations) than geography does, but somehow the world map has established itself as decoration of choice for juku classrooms.