My high school system used to convert those to zeroes, just FYI. Anything that was nonnumeric and not a fixed set of "aliases" (for example, "pass" might mean 100% if the teacher set it up that way) became a zero.
How about using an integer where an integer is needed and false where no grade has been produced, with the teacher and student UI showing 'Not yet graded [x]' rather than using magic numbers?
You know it