Morticia, Lox's rule stands in Canada too, but I don't know about what is taught in the U.S. regarding grammar.

The only exception I can think of, off hand, regarding the possessive is the word "it", for some reason or other. Dam! English must be a difficult language for those just learning. "It's" is the contraction of "it is" and "its" is the possessive. And I always forget which is which when I'm writing.