Here is an English usage worksheet on sorting out the use of alumnus and alumni. If you’re so inclined, it would take only a moment to explain the inflections at the end of these Latin words and in so doing plant basic knowledge about inflected languages in students’ minds.
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.