I always cite my bad Digimon Mary-Sue as the first fic I wrote, but I just suddenly remembered that the first fanfic I wrote was actually for the game Oregon Trail! I turned it as a class writing assignment  in fourth grade. (I remember I wrote that one of "my" daughters died while we were fording the river because that's always what happened to me. I even wrote in a run down of how many chickens and yards of muslin we lost.)  

Did you ever write fic or turn in something fannish as a class assignment?