Yumari-chan wa Karei ni Ohana wo Tsumitai

Yumari Kawaya, a serious honor student at school, is secretly obsessed with the pleasure of defecating in the "bathroom"... When Yukinami Takaga, a big-breasted kuro-gal who doesn't get along with Yumari, witnesses her beatific face after defecating, the door opens to a new fetish!

(Source: Shonengahosha, translated)

Note: Originally serialized under the name Otoilet Ikusa Shoujo: Yumari-chan (おトイレ戦少女 ゆまりちゃん), the title was changed for the volume release.