It's this story of a manga: https://exhentai.org/s/8e5c79ed10/476862-83

It's a cliche plot, I posted because she is cute but I'll explain
Guy is a skilled math teacher filming girls peeing. The story starts by him fapping to the posted girl (Mizuno-chan) getting caught peeing, which he faps to in his house. The next day he notices her attitude is weird so he checks out toire after school, and finds out the camera is gone
Mizuno-chan appears asking if the camera is what he is looking for, he begs her to not tell about it to anyone, and she makes him have sex with her. A girl coming to toilet while they are having accidentally reveals Mizuno-chan loves sensei! (Something like "Mizuno, did u see sensei? You love him so I thought you'd know", while they are ecchi'ng in cabinet)
The story has a happy ending where she is proposing marriage to sensei and he says "I'll be the one to propose", yay!