It could be that the transexual character is not a girl of similar age to the MC but one of the older women, i.e., a teacher (Cindy, Harrison, Marla, Coach Hill) or non-academic member of the college staff (Karen). It seems more plausible to me that the trans character would be older because I understand that it takes several years to transition fully from male to female, adjust fully to the new gender, and become comfortable and confident enough in yourself to successfully and happily blend in: I doubt that any of the student girls would have been able to embark on such a journey as early as their mid-teens, possibly even younger, to have had time enough to complete the process considering the relatively young age of the female pupils now.
(Not that I'm an expert or anything.)
Just sayin'.