Kono Kyoushi, Zettai Wazato.

The beautiful Mio Saeki may seem like a proper young lady. But when she's in the language-arts classroom she acts like an old man, eating squid sticks and talking about horse-racing. Only her teacher Shirokawa knows her true nature and despite his slightly sadistic teasing, they get along so well that she finds herself falling for him. Note: Includes 4 extra chapters.