I totally lost this source but this is what I remember about it
I Remember that it was about this guy having a relationship with his art(?) teacher and if I remember correctly there was a point where the class or the boy goes to a sort of exposition or a premiation for something realted to the art (edit: it could maybe have been about calligraphy), after this sort of premiation or exposition, the teacher and the students have sex at the teacher house. At some point a girl of the class start get suspicious about the mc and the teacher and she starts asking about having sex with the mc or blackmailing him, or something similar.(I remember also that the teacher was taller than the MC, she had dark hair and big boobs, in the chapter of the art expo(?) she wore a different dress) (the other girl had pigtails maybe, with hair maybe brunette or orange).
I'm pretty sure that there were minimum 2 or 3 chapters.