Kono Kyoushi, Zettai Wazato.

Mio Saeki appears to be a well-mannered young woman, but in her language class, she behaves like an elderly man, eating squid snacks and discussing horse racing. Her teacher, Shirokawa, is aware of her secret identity and teases her in a mildly cruel manner. Despite this, their relationship is close, and Mio develops feelings for him. The story includes four additional chapters.