Historias de Nagisashitoa

It's 10 years after Koro-sensei's death, and Nagisa is sent on a mission to track down and eliminate a dangerous group of people and their manipulative boss. It's 13 years later and, Nagisa hasn't been seen since. Karma and Rio's daughter, Rei discovers something that could change everything for the better or for the worse, but it would take all of class 3-E to find an agent who is 13 years MIA. I DONT OWN ARTT NOR ASSASSINATION CLASSROOM!

Nagisa and Karma haven't seen eachother in a year since middle school ended. They are now both in highschool. Karma transfers to Nagisa's school. Why did he?