The students able to reunite with Korosensei and trying to find a solution to save him. Unfortunately, their celebration of Korosensei's birthday is interrupted by Korosensei's former pupil who is seeking a revenge all these time.
The second generation of the "God of Death" has been modified and possesses skills twice powerful than Korosensei. The battle between them is so powerful that none of them including Karasuma can offer an assistance to Korosensei. The revenge is going to another level when the monster intends to kill the students as well.
When one of the students decided to protect Korosensei and the decision costs her life, Korosensei feels extremely upset and leads him to fully utilize his maximum power for the battle.
Finally, it's the time for the students to fulfill their teacher's wish.
I can't believe tears flow from my eyes. It was an unexpected ending or maybe I just don't dare to expect anything for the ending.
More reviews can be found on Goodreads: Assassination Classroom, Vol. 20: Graduation Time (Assassination Classroom #20).