If one had the grit, one can argue that a class in postmodernism can not be graded. After all, if the final exam grade the professor gives you is only his version of the truth, it's not automatically valid for you! His version of the truth shouldn't really matter to you. It is only his opinion of your performance. Therefore, a bad grade from the professor is not valid in your construction of reality! After all, you define yourself as a member of a different local community: that of a college student. Since truth and reality only extend to the borders of local communities, they are outside your local construct.
So if you are a gutsy person, show up at your professor's office, and joke that you deserve an "A" because if they gave you a lesser grade it only represents their opinion of the true value of your exam paper. Hey, we're not supposed to be making truth claims here are we? How can a professor teach that there are no truth statements one minute, and then turn around and hand over the grades to the dean the next minute, declaring them to be a true analysis of your performance? Did they just forget that they told you that there are no valid truth statements?
Having shown that postmodernism is not a practical theory for colleges to teach, since it contradicts their grading system, you deserve kudos just for listening to the nonsensical prattling of Post World War II French philosophers all semester. After all "I'm paying a lot of money to be educated! Give me something I can use in real life. Don't waste my time and money."
Unfortunately, although it is philosophically sound in principle, you'd have to know the professor pretty well to joke around with your grade. But they should respect the fact that you are intelligently engaging with the subject.