“I’m an alumni.”
Really? How many of them are you? What was it like being a students?
The forms of this word don’t really have to be difficult. Of course, you could simplify everything and say “one alum, two alums,” but that’s cutting corners. And if you want to sound like an ignoramus, try using “one alum, two alum.”
But here are the true-to-Latin forms:
(or gender neutral)
Of course, you are free to do whatever you wish. If you freestyle, though: others might believe that you are no alumnus/alumna from anywhere.
Image source: http://leedsunicareers.files.wordpress.com/2014/06/flickr-4647211575-hd.jpg