Shouki

The story takes place at a highschool (featuring all kinds of pupils, including robo- and catgirls).

The players character (Kichijouji Kai) loves to play mahjonng and wants to become a regular member of the mahjonng-club of the school.

However, at that day, when he went together with his classmate Ekoda Shoko (who secretly seems to have a crush on him) to the mahjongg-club, they arrive at a scene of horror:
All the regular member of the club lie around in the room, unconscious. The tables are all broken in half. Everywhere in the room, there are mahjong-tiles scattered on the ground. Some people are even pinned to the ground with counting-sticks.

So, suddenly, the whole mahjonng-club is reduced to him and Shoko.
Because two people are not enough for a decent game of mahjong and in order to rebuild the mahjong-club as well as to investigate, who did that to the former members, Kai and Shoko start to challenge members of other clubs in mahjong. If these other club members lose, they have to join the new mahjong-club and have to tell them everything they know or suspect about the incident.

[From this forum post]