Well, we don't have Fs here...but I remember one of my year 7 italian assignments being marked as "Ungradeable" (UG) so 12. Don't get me wrong, i was good at the subject...I just got lazy.
The lowest grade you can get in my school is a B. (Equivalent of a D in my country/F other countries.) I was about 7, I guess. When you had your first embarrassing moment.