I read the chapter where Najimi was introduced and it mentioned that they got transferred a lot because of their parents' circumstances so... Maybe that would get touched upon in future chapters.
In case anyone is curious the bat smack on the butt plus the Thai kick is a gag from the annual Japanese comedian special show "Downtown no Gaki no Tsukai ya Arahende!!" where a group of famous comedians spend a day playing a themed punishment game where if they laugh they get their butt smacked. They bring in famous Japanese people for skits and the Thai kick is one of the reoccurring gags.