Sorry to go back to your post, and I admit I have not yet played very far the game, so I guess it uses a kind of "USA model". But if you used as different country/systems as reference for your expectations, than 1 or 2 years could be enough.
In general in Europe (but AFAIK in Japan is the same) students in high school (which arrives to cover full legal age, not just age of consent) are staying in the same class together (even when doing e.g. physical education, they will often do it in parallel, even if they do it separately between male and female, or if they go to the lab, they will go all together and move to the lab, since it is a specialised room), and the teachers for the different subjects rotate in the class.
Of course there can be cases of students failing the grades to pass to the next year, thus not being anymore with the same people in the same class the year after, changing class or school, but that is the general rule.
That means that in one or two years can develop even very good friendship (or strong antipathies) with their classmates, as they really are classmates.
