This is a weird kind of reunion - in our country reunions takes only class where you've been to, your year - bcs why would anyone want to the reunion where is people that graduated year(s) before you? Bcs it will be crowded and a lot of people you dont know and in the end this kind of reunion transforms in a bunch of little groups, which seems absurd. I guess, this situation was made by author to add this drama - "he is older than her, he leaves, then after 13 years they reconnect at the reunion". (But maybe I'm wrong and in authors country this is normal, to celebrate graduation for different years - we've got reunions like 5 years graduation reunion/10 years graduation reunion - this kind of staff).