@Deathkillz:
About atmosphere: the lighter atmosphere is too be expected since they moved to a high school. However, that at the same trime makes it darker: in all that happiness and noce looking world, in paralel there is a darker and cruel world of assasins and mafia. As the OP shows, all those people (including Reiji's and Ellen's school firends) have no idea that those three nice looking people (Reiji, Ellen, Cal) are in fact cold-blooded assasins. And nobody especialy knows that there is a war brewing between two top assasins from one side, mafia's from the other side and another elements from the third (after all, Inferno and Cal didn't come to Japan for a vacation). And the watcher is left wandering if some of those innocent people will end up being pulled from their kind world into this cruel and dark world. If someone innocent does get involved, will that person be able to return back... or will that person remain scarred for the rest of its life?
Looking at it this way, the light atmosphere becomes a good thing, IMO.
Actuall, it is possible. A lot of people can grow up fast.
Also, this is nothing compared to Ellen who in part 1 was a 14-years old girl. If a 14-years old girl can look like that, then Cal in her 16's can look like this.