From up above in the top comments:
series order-
1.Doukyuusei (NAKAMURA Asumiko)
2.Sotsugyousei
3.Doukyuusei & Sotsugyousei Fanbook (Side Story)
4.Occupation to Beloved
5.blanc
6.Sora to Hara (Side Story)
I think the rings were an extra mixed in there somewhere. Mangago isn't the greatest about putting stuff in the proper places.
Ahh! I actually just found it. It's in Graduation Certificate!
Thanks you so much you save mah lifee (づ￣ ³￣)づ
You know how there's all those pictures just above the chapters list? Click on the one that is labeled "Graduation Certificate"
can someone tell me which Manga Kusakabe and sajou exchanged rings, I'm a bit lost the story