Write as:
Select *, --name all columns you want
case when Percentage < 80 then 'B'
when Percentage < 70 then 'C'
else 'Add the grade which should be default when above 2
case conditions are not met'
end as LetterGrade
from (
Select *,
100 * COUNT(CASE TurnedIn WHEN 1 THEN 1 ELSE NULL END)
/ COUNT(TurnedIn) AS Percentage
From Table1
When ..) T