Nuances of Failure

If you have grades of D, EA, F, FN, EX-D, EX-F, or I in any of your classes,…

I’m reminded of things I have read or heard (probably from TV), that the Japanese have multiple words for rice, and Eskimos have multiple words for ice (or was that snow).  And, I did read in Frank Herbert’s DUNE, that the Fremen had multiple words for sand.

I only know what a D or F is.  We didn’t have an E.  Does FN stand for Failing Naturally?  And does EX stand for Exceptional?