Graduation Time Limit

The series

A popular English teacher, Mizuguchi Risako has been kidnapped three days before the high school's graduation ceremony. Four high school seniors received a mysterious challenge letter, asking them to solve the mystery of the kidnapping case within 72 hours. The four students are Kurokawa Yoshiki, his childhood friend Takahata Ayane, sports all-rounder Ogita Junpei, and school's beauty contest winner Komatsu Mio. The four students were wondering why they're the only ones who received the letter, but later gradually found out the "other face" of Mizuguchi which she has been hiding.