Funny. In the old days (back in the dark ages when I was a middle-schooler) it worked like this:
90-100% = A
80-89% = B
70-79% = C
64-69% = D
<= 63% = F
But I understand that most schools now use 70% as the “You got an ‘F’ kid. And ‘F’ stands for ‘Fail’ you stupid frakin’ loser” breakpoint.