Bishoujo Janshi! Sparrow Girl

Feeling suffocated by a world without change, Otowa, an honor student attending a school for young ladies, meets Kotoriko, a pretty girl in her class, and follows her on a night out on the town.
On the way there, Otowa sees Kotoriko wearing a mask and a dainty costume, and throws herself into a high-stakes mahjong match.