It seems that Tengoku, a school well-being counsellor, does more than just dish out health advice to students. The rumour is that you can lose your virginity if you visit the school's infirmary. The hapless Japanese teacher, Aoki, goes to investigate…
Oni to Tengoku is a completed manga by Aga Naomi, first published in 2016. It blends psychological, romance and boys' love. You can read all 13 available chapters of Oni to Tengoku online for free on MangaPulse; the newest is Chapter 10.5, added 2019-10-06. The series is finished, so you can read it from start to end in one go.