Mr. Hiiragi's Homeroom

What is Mr. Hiiragi's Homeroom about (plot, summary)
Ibuki Hiiragi works as an art teacher. He began teaching at his current high school two years ago. He is the homeroom teacher of 3rd Grade A Class. 10 days before graduation day, Ibuki Hiiragi stands in front of his 29 students, including Sakura Kayano, and tells them "From now on, you are hostages."
Ibuki Hiiragi works as an art teacher. He began teaching at his current high school two years ago. He is the homeroom teacher of 3rd Grade A Class. 10 days before graduation day, Ibuki Hiiragi stands in front of his 29 students, including Sakura Kayano, and tells them "From now on, you are hostages."