I told politely requested the reunion committee to go pound sand kindly remove me from the mailing list when they sent a card alleging that it had been a quarter century since I graduated. Imbeciles! Terrible at math.
I am told, by those who still like to go to such things, that the groupings are now by decade, for the reason you note.
I still say that with every one of these things they announce, they just get worse at math.
I think you're on to something with that. Class lines got blurred in the (musical) ranks, for sure.
Funny in a way, but I know some 80 year-olds for whom the reunion was given for the entire school, as in, absolutely anyone who ever attended it, ever. The reunion wound up being 8 people around a table.