I ran into it a few months ago and am almost positive I might've found it here. It's art is stylized less like hentai and more like a coming-of-age story as the girl is a very average-looking, ordinary schoolgirl with hair on the shorter side, maybe. It starts off with she and her family having moved to a new town, wherein she feels isolated and unhappy. Part of the reason being that she has a long train ride to school everyday. On these daily train rides, there's an older man who begins to do sexual things to her that she reluctantly enjoys, and while she freaks out at first, she just kind of goes with it eventually and it becomes part of their daily routine. There are scenes when she's home with her parents, hiding the secret, where she contemplates her feelings on the matter and of how she feels like life in the new town isn't so bad anymore, etc. In the end, she winds up skipping school one day to have a day of sex with him. If memory serves, even the cover art doesn't appear all that overtly sexual.
Thanks, in advance!