Assassination.classroom.s02e25.future.time.1080... Here

“Now then, class — let’s begin today’s lesson.”

That specific episode (Season 2, Episode 25) is titled (or similar, depending on the fansub). It is the series finale of the entire show. Assassination.Classroom.S02E25.Future.Time.1080...

If you are about to watch this file: The final line, spoken by Nagisa to his own students seven years later, is: “Now then, class — let’s begin today’s lesson