Sensei, Gaman Dekimasen

Kiryuu's an Alpha teacher in a school full of Betas and Omegas, he is extremely popular with the students for his neat face and friendly personality, and also because of his unique constitution that is unaffected by pheromones. When an Omega student, Ao, gets his first heat, Kiryuu helps him. But from that day, Kiryuu's behavior changed...

(Source: Ichijinsha, translated)