"Assassination Classroom" season 2 signifies the death of Koro-sensei's death from the hands of his very own students. Spinoffs are under way that is speculated to be a combination of season 1 and 2. That should be released after the "Assassination" Classroom: 365 Days which is a film adaptation.

Koro-sense might have been so tough to his students when imposing discipline in their training that drained them out - but such reason to kill the master is not justifiable. The trainer got back his cruelty when both Yanagisawa and Reaper combined forces to exhaust him. The rest of the class decided to finish him off when he was already weak and hopeless.

In Assassination Classroom season 3, Koro-sensei is reincarnated - but not the original master's character. This time, he is the so-called demon king.The important event in this season happens elsewhere and not just in the classroom like before. Sword and sorcery are Koro-sensei's means in carrying out his demonic mission. The plot follows Koro-sensei Q! ( Koro Teacher Quest! ) by Kazuki Watanabe in Saikyo Jump back in 2015, as featured in iTechPost.

"Assassination Classroom": 365 Days released last November 19 will be continued and will focus on the Nagisa and Karma bond. The dark side of Koro-sensei will also be revealed. 12 episodes will comprise the spinoff lasting 10 minutes each. 

As of this time of writing, season 3 release date is not yet known. The Weekly Shonen Jump should be a reliable source for it when it is made availableF, according to The Bitbag.

Jobs & Hire previously reports, that a familiar character of "Assassination Classroom" Ryosuke Yamada (Edward Elric) who is featured in "Fullmetal Alchemist" Live Action Movie trailer might lead other characters in making appearances in "Assassination Classroom" Season 3  but in their anime versions.