The most powerful and enduring conduit for folkloric items such as this spelling rule is not your third grade teacher, or your resident linguist, but that awe-inspiring group of children just a little older than you. You know the ones: older siblings, older neighborhood kids, that one older girl in a group of younger ones. They use old sayings like "curiosity killed the cat," they pass down jumprope and hand-clap rhymes, they teach games like M.A.S.H. as well as study tips like ROYGBIV and, yes, "i before e." Then, armed with new knowledge, these children pass it to others even younger. The flow is endless.
Which is not to say that it is inviolable. I have had many arguments over the correct categories for M.A.S.H. and I have been shocked at the discovery that a girl from the Midwest was saying the "Miss Suzie" chant wrong. Things change as they are passed down. Origins are forgotten. Words are forgotten or misheard. This thing called language is like one giant, universal game of Telephone. Who knows what will come out on the other end.
This is all my way of telling those teachers, grammarians, and lingusts to chill. I'd like to say the same to the French, who hold their language so dear that any small change requires one to jump through major bureaucratic hoops..sometimes on fire. Why try to force or supress such a natural phenomenon as the changing of language? Let's just go with the flow, folks. Now, who remembers Mary Mack?