Yamada is a junior high school teacher who spends his days being mocked by his students, including Morita who nicknamed him "Toroyama''. The other party, Morita loses her virginity to her stepfather, her mother's new husband, so she no longer trusts adults and decides to take advantage of them. In order to save money for her future independence, Morita works at an "adult services" business run by her stepfather. One day Yamada calls for a "health service" delivery girl, and she turns out to be his student Morita. Yamada holds Morita in his arms as if to rape her in order to relieve his daily frustrations, but it turns out just to be a roleplay done by Morita and he fell for it. Since then, Yamada has become addicted to Morita's body and so he becomes a regular customer. Like with the other adults, Morita didn't trust Yamada, but as they got closer to each other, she began to open up to him a little more. Their relationship at school remained the same, but in their hearts a changed had definitely happened.
Man, this is the one series from malcorond that I most want to see translated. I love Saya-chan and Fat-sensei (Toroyama) together. And Chapter 3 is where they have a 3P with another girl.
There's a chapter 2.5 for Saya-chan and Toroyama that was also published/translated in "Delivery Girl Picture Book <Compilation>" --> https://e-hentai.org/s/777389a15a/2885484-96