One of the first things you learn in Latin class in high school is the word “alumna/alumnus”. There are a few reasons for this. The first reason you learn it is because you’re in high school, and you’re interested in graduating, and “alumna/alumnus” features in a lot of the early, simplest sentences about students in school, or students who have just graduated from school.
Another reason is that alumna/alumnus are pretty easy & regular nouns. “Alumna” is first declension female gender meaning “former student” or “graduate of”. A single female graduate is an “alumna” — two or more female graduates are “alumnae” which is the plural form.
“Alumnus” is second declension male gender meaning “former student” or “graduate of.” A single male graduate is an “alumnus” — two or more male graduates are “alumni”.
Here’s the thing that struck me about Kavanaughs yearbook entry. There is no such freaking word as “Alumnius”. It does not exist in Latin. And normally one would just say “oh, they just were unfamiliar with Latin,” but this is a Catholic school, Georgetown Prep, where even today the website proudly states that students are required to complete two years of Latin, Latin I & Latin II.
So if you are a Latin student, by second year Latin you know the correct spelling of “alumnus”. It’s “alumnus” not “alumnius”. And all of the potential endings of the noun are drilled into you, because that’s what you memorize. “ius” is not a first or second declension noun ending.
“Alumnius” is not real Latin. It’s not a mistake, because these boys knew Latin (the Catholic school kids in the ‘80s took a lot of Latin & dominated at the Latin conventions because they spend so much class time on it — yes, there were Latin conventions). So was it a typo? Probably not, same reason. Nobody typesetting the yearbook would make that mistake, it would be like spelling “dog” “doig”. So it’s probably another inside joke like “FFFFFFourth of July”.
Translating “Renate Alumnius” out of joke Latin we get “one male” (-us) “of many male” (-i) “former students” or “graduates of” Renate.