Hyakkaou Private Academy is an institution and the main setting in Kakegurui. It is a school that Yumeko, Ryota, and Mary attend as well students from powerful families in the political and financial world.
Hyakkaou Private Academy was founded 122 years ago in Japan and for many years since its inception, was a school where students from powerful families in the political and financial world attended. It is a school where students are not evaluated by studies or sports, but through the social status they possess. This is because studies and extracurriculars are not necessary for people who are destined to have power over others. For them it is important to be well versed in tactics, reading an opponent, and the strength to grasp victory which are abilities they have to prove and obtain through gambling.
Two years prior to the start of the series, a student changed the school tradition after winning a gamble from the former student council president. This caused the creation of the pet system. The pet system consequently increased the discrimination among the students. Those who weren't good at gambling were oppressed. They were seen as housepets and were constantly bullied. Meanwhile, the victors were praised.
Gambling at Hyakkaou Private Academy is governed by the student council. It is known that the student council members use their powerful influence to collect money from students of Hyakkaou Private Academy. Payment of it is called a "donation", where it is optional but is practically obligatory. Hyakkaou Private Academy has a ranking system which can be given to students that have failed to donate to the school. Those under 100 of the 3,000 students or who have accrued a massive debt that they cannot pay are recognized as a "housepet". If a student is a male, they will be called "dog" (pochi), if the student is a female, they will be called "cat" (mike). The hierarchy in Hyakkaou Private Academy is absolute where no student can go against it, and if they do, they will await the student council's reprisal. If the student wants to rid themselves of their "pet" status, they have to donate but it will be impossible if the student has either a massive debt to pay off or no money to pay their way out of their status. The hierarchy is described as a perfect dead end which is known by the students as something that is extremely hard to escape.