That's a fantastic point I'd forgotten: that elementary school teachers once were (and in some cases maybe still are) supposed to teach pupils how to judge books by their covers. In the U.S. it was part of the "whole language movement" (which seems to be losing steam now but was all the rage a couple of decades ago).

It must have confused some children to be told "here are the clues you should look for on covers" and then to hear from, say, their parents, "Don't judge a book by it's cover." I love your comment!