Tsukiyo ni Otome wa Tegami o Nurasu

It's just before 9 PM and the classroom is dark. A month ago, Mayumi started receiving alarming anonymous letters. She teaches at a school for high-class ladies, and suspects the culprit is a student as well — but the contents of the letters are both strange and obscene, detailing dissolute, promiscuous lesbian sex and an unmistakeable, base interest in Mayumi herself entirely unfitting for such ladies. Determined to set the writer straight — or at least get them to leave her alone, as she is quite put upon by the barrage of letters — Mayumi sets out to confront the writer. She has summoned them to the classroom; all that remains is to wait for them to arrive, if they dare.