あいらんど・りぞーと-決闘沖姫島分校-

Main character transfers to a school on a resort island because of his father, who gets appointed at work to be responsible for island development. However, opposition campaign has burst out to prevent reconstruction of the island, and the protests wave reached the school as well. Supporting the the right to develop the island, protagonist enters a surrogate war at school where the battles are held via mahjong game.

[From vndbreview]