Otona no Omocha, Bosshuu sarechatta! Tsukaikata Oshieteyaru yo

Mr. Hiiragi's a popular teacher, favored by students for his enjoyable lessons. Chika, the class representative, looks up to him, and even has a crush on him that was supposed to stay secret... However, one fateful bag inspection day, their relationship takes a sudden turn. It should've ended without incident, but inside Chika's bag is something that wasn't there before! Mr. Hiiragi keeps her after school for a special lesson on how to use it...

(Source: Renta!)