Okay, the premise has me puzzled. Does An’s school start classes later than the Azeno school? I can’t think of any other way it makes sense for her and Shunpei to be taking the same bus every day, if Azeno is a fair distance from both her school and Shunpei’s house.
That said, the dynamic of a class that’s been together since they were little is something I haven’t really seen explored before, and it’s one that’s a little familiar to me — my old yearbooks show 41 kids in my grade in kindergarten and 25 in 10th grade (counting me), and 17 of those are the same by my count. It’s even more striking if you compare kindergarten to 5th grade: only 7 of those 41 left, and 11 more were added, for a K-5 overlap of 34 kids.