It's one of those hentai series that also gets a cut-down TV broadcast. Family-friendly episodes (aka. "regular edition"): 3 minutes each. 18+ streamed episodes (aka. "complete edition"): 6 minutes each. These have different voice actors, incidentally.
These shows aren't usually much good, even by hentai standards, but they keep popping up because it's a good way of grabbing eyeballs. This one's title translates as "25-Year-Old High School Girl: I'll Do What Can't Be Taught to Children".
Our heroine is Hana Natori, who's going back to school. She's going to pretend to be her cousin who refuses to turn up to class and will have to repeat the year if Hana doesn't do something. (Why would that be bad? If someone misses lessons, it's not doing them a favour to make it look as if they haven't. Speaking as an ex-teacher, I think Hana's letting her compassion override what would actually be good for her cousin.)
Anyway, Hana's a young-looking 25-year-old. She thinks she'll be able to get away with this. Unfortunately, one of the teachers (Okito Kanie) is an ex-classmate of hers from high school.
Result (in the "complete edition"): sexual activity in every episode. Kanie's a bit of a predator, although also concerned about Hana and keen to protect and help her. He'll give her assistance and private tuition. (That's not a double entendre. Hana wasn't the brightest student even seven years ago, so she's at sea with a lot of the curriculum. Kanie teaches her.) He'll even get jealous when a bottle-blonde bad boy (Naoyuki Aida) starts taking an interest.
The question with these TV/hentai tends to be how rapey they are. Twelve episodes suggests you want a story arc, which unfortunately is liable to be "no no no no yes". Here, though, it's... uh, not too bad. Hana doesn't actually tell him "no". She's definitely not soliciting Kanie's attentions, but her reaction to him pushing his luck is usually just flustered embarrassment. She doesn't even get angry at him, although she's capable of it (ep.11 after they'd said too much to each other and he definitely went too far). On that occasion, she cries and kicks him out.
Technically Kanie's in a position of authority, but in practice that doesn't apply to Hana. She could (and should) have walked away. She didn't have to be there. Nonetheless, she keeps hanging around Kanie for advice and, um, action.
It's a romance. A rocky one, admittedly, but a romance. They admit that they love each other at the end, although Kanie's the kind of sadist who'll forget to mention that he'd locked the door before having sex with Hana at school in ep.5. She'd been thinking anyone might walk in. Then, in ep.10, he's even more malicious.
Is it good? Not really. It's sort of okay, though, which is a huge improvement on some other shows like this. It's a bit boring, but that's better than "creepy" or "out-and-out rapist". (In "Shuudengo Capsule Hotel de Joushi ni Binetsu Tsutawaru Yoru", a woman keeps being molested by her boss. In "Sweet Punishment: I'm the Guard's Personal Pet", a woman in a men's prison becomes the sex toy of a manipulative, sadistic warden.)
Kanie's walking a fine line, but he's also being way too soft on Hana's highly dubious plan. He's helping her! I disapprove.