Kono Kyoushi, Zettai Wazato.

The beautiful Mio Saeki seems like a quiet young lady but when she's in the Japanese prep room 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. (Source: MU) Included one-shot: Volume 4: Futari no Houkago