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: Shonenga