https://www.dlsite.com/girls/work/=/product_id/RJ01133748.html

The page numbers at the bottom are a bit odd at the beginning, with 57 being between 1 and 4, but that's the order it was in. I guess the artist changed their mind midway and forgot to update those page numbers, only changing the filenames. Also, despite what the description might imply, there's really no NTR or anything here. You'll find out as you read on, it's pretty surprising.

Description:

Chisaki fell in love with her private tutor, Shuu, and they have been engaged in a physical relationship ever since. Their lessons include three periods, each lasting an hour. Her parents don't return until late at night, so they are free to do whatever they want for the remainder of their time alone.

By chance, her childhood friend Asahi happened to have a day off from club activities and dropped by, only to witness the sordid scene of the girl he had given his heart to with her tutor...