Kyouko is older, Kaori I think is around the age of Mikoto and Takeru, I always assumed Rinko and Sora are in middle school or the first year of high school but it's never stated out right of course.
I'm pretty sure the main 3 and Yamato are also supposed to be in high school.
I always assumed Inyou 1 is in Winter/Spring, shoku is Summer of that year, etsu is 1 or 2 months later, Kyou is the next summer after that and goku 1 year after (although not during summer, based on their clothes). That would mean the whole elapsed time is about two years, but that of course is based on what i remember of the games so it could be inaccurate, I could check in a relatively short time because it's always stated in the intro when the game takes place, I'll see if i can do it.