while True:
    
  grade = input("What is your grade? [A, B, C, D, F, or X to quit]: ")[0]
    
  if grade == 'A' or grade == 'B' or grade == 'C' \
     or grade == 'D' or grade == 'F' or grade.upper() == 'X': break 
              
# now do something with "grade"