Kakegurui Futago

A prequel to Kakegurui is set a year before the main story's events, and is centered around Saotome Mary.
Saotome, a bright young girl born into an ordinary family, has passed her transfer examinations and has started to attend Hyakkaou Private Academy. It's an elite school with an exquisite twist - gambling has become a central part of the school's rules. Saotome adapts quickly to the school's outlandish system, and soon succumbs to the gambling mania that she comes to be known for.