A Strange Sensory Zone

A few months ago, rumors spread at the school about a love affair between Takasu Reiko, a female biology teacher, and Yamada, a physical education teacher with a wife and children. It was rumored that Yamada toyed with Reiko and then abandoned her, and Yamada was transferred without knowing the truth. Since then, Reiko's strange behavior has become conspicuous. One day after school, when a female student opened the door to the biology room, Reiko exposed her underwear and pressed something black against her crotch, making her look ecstatic. What the heck is that dark, strange thing...!?!