The story focused on students of Kuro Sensei may be over in "Assassination Classroom" but it doesn't mean the alien teacher's story is already over. Kuro Sensei's story continues with anecdotes on what happened in the past that was not included in the teacher's official manga. The special extra chapter is already available for reading in Manga Stream.
On a bar, assassins are gathered together learning more about the whereabouts of the teacher. Through their stories, it was revealed that Kuro Sensei dedicates a day where he could focus on what he really loves the most. The assassins saw him browsing through books, medicines and even adult magazines that he has collected through the years.
The chapter end with Kuro Sensei showing up in the assassin's bar. It was then later indicated that Kuro Sensei's premise in showing up in the specific bar is due to the fact that one of the bar tender looks like one of his students.
The upcoming chapters of "Assassination Classroom" will focus on the timeline where the teacher is no longer interacting with the students. Since there are three more chapters, Koru Sensei's adventures will be revealed especially when his not interacting with his students. In the latest chapter, it's clear that Kuro Sensei has an objective that would ultimately help his students. Of course, his hobby will also come into play no matter how weird it is to his colleagues or students.
One of the intriguing stories of Kuro Sensei's story is the looks of the new unnamed character. According to user MaxXzoe from My Anime List forum, "so that's what Nagisa looks like if she was a real girl... at first I thought it was Kayano, then upon closer... mind blown..." Kuro Sensei is definitely flexing his muscles to help his students to bring out the best for his students even when he's not as school. The chapter shows that he is actually prepare everything for his students after he is gone.