You are within your rights to disagree, but you're still wrong.

You'll find that some dictionaries are more conciliatory than others, and the Random House folks seem to lean in that direction, possibly more than most. Personally, I find that ``alum'', assuming that you're placing the emphasis on the second syllable, to make it simply an abbreviation of ``alumnus'', breaks the rhythm of the language.

I would also argue that ``accepted informal'' is exceedingly close to an oxymoron, but that's a different story.