Baka-updates link:
Synopsis: Set one year before the main story and centred around Saotome Mary.

Saotome, a bright young girl born into an ordinary family, has passed her transfer examinations and started to attend Hyakkaou Private Academy. It's an elite school with a twist: gambling has become a central part of its academic activities. Saotome adapts quickly to this outlandish system, and will soon succumb to the gambling mania that she comes to be known for in Kakegurui.

The first chapter was scanlated but it looks like the group that's doing it is not active anymore.
I have the raws for the first vol here provided by another forum poster on this site:

I've helped out Deadbeat scans as a redrawer before so I'm willing to help out if this gets picked up!