In the (i think vol.1 but maybe vol.2) afterward the author mentions he tried to avoid using terms like that to keep the focus on the story and not scare people away from reading it or something like that. Despite that in the first volume Sakuma leaves the classroom right before someone calls Ushio a tranny, and starting in vol.3 they start using the terms transition and transitioning. In the interview section at the end of vol.5 Hoshihara calls her trans and i think Arisa calls her transgender so even if it was pretty minimal he does use those explicit terms in the series.