Hmm. When I read your comment I thought it was a funny variation on Strunk and White's "Omit needless words": funny because it goes them one fewer, variation because it means the same thing. So if you're saying it was intended as "don't use obscure words", then your teacher pulled a fast one. It's probably better (i.e. more age-appropriate) advice for smart fourth-graders, though, than the other one. (Though maybe not. One wouldn't want to inhibit a nascent vocabulary!)

My understanding is that it was intended as a variation on "omit needless words," but that said, "prolixity" is technically the wrong word for that job (as prolixity is more concerned with total word count, and less with individual word choice). That said, I think both its message, and its wording, served to teach both lessons: one directly, and one indirectly.

Incidentally, the same class also served as the first of my many introductions to Strunk & White. We were way too young for their book, but it never hurts to start early.

You reminded me of something I heard on NPR recently. When E.B. White was old and couldn't remember things, his son would sit and read to him. Sometimes he would read pieces that White had written, and White would say, "Who wrote that one?" And his son would say, "You did." And White would say, "Not bad."