出典:Wiktionary
The ordinal number refers to the number of years since graduation, not to the number of reunions. In the US at least, reunions at 5-year intervals are common, particularly 5th, 10th, 20th and 25th.
a class reunion
a class reunion
the teachers' conference
有閑階級.
a social gathering―convivial meeting―a friendly reunion―(米国にては)―a sociable
地主階級.
授業中に