What is this manga about a high school class that turns into a kingdom?
I believe it is Oukoku Game.

Synopsis from mangaupdates (emphasis mine):

At the end of Meiji Restoration, a girl saint was crucified and burnt alive under persecution.
A century later, Katori and six childhood friends are summoned to her world beyond the nirvana, where they receive the Great Commission: Build a kingdom, and make disciples of all nations.
At every midnight, each of them are randomly given a role out of King/Queen, Noble, and Peasants. As the lowly must obey a higher-up in the hierarchy, can they fight and defeat other nations to conquer the world--before infighting brought up by their own hidden desires breaks them apart?