In practice, you're simply criticized for not discussing the context and structure through which you view the work. Arguably that's what critical theory is as opposed to other frameworks of criticism, namely structural theories like marxist and many tracks of feminist thought. Not only is this not a problem at all in practice, it isn't really relevant to the topic at hand because "postmodern literary analysis" doesn't generally exist at the high school and survey level.
The only criticism I think is worth teaching students is that they are being decieved and misled by their instructors and their job is to figure out how and why.
I object to this premise.
Trivial counterexample: Is it impossible to evaluate the quality of a novelist because we don't have a common understanding of the plot of a novel until after I've read it? Clearly not.
Even in a post-modernist literary paper you can still judge the writer's cohesiveness of thought and clarity of argument.
Edit:
>The correctness of the content is subjective. If the correctness of the content is objective, and mutually understood, then any confusion added during the writing is bad writing.
I also don't understand how post-modernism comes into it. Is talking about the theme of insanity in Hamlet any more objective/correct than writing about the influence of race in Moby-Dick? If so, how?
When professors or employers complain that their students or employees can't write well, and lament the quality of public education that caused the ineptitude, they are talking about writing as a means of communication. They aren't talking about any of the mentioned parties' inability to produce good novels.
A novel is good if you enjoyed reading it, even better if it has re-read value. A good novelist produces good novels. Whether or not you enjoyed reading the novel actually has nothing to do with whether your understanding of the plot is the same as the author's. In order to confirm that the novelist was effective at using writing to communicate information, that information would have to be known as ground truth on both sides of the channel. As an imperfect example: if you had a chat with the novelist, and it was revealed you had a different understanding of the plot, then they couldn't be that clear of a writer, even if you enjoyed the novel.
You can't evaluate how good of a writer someone is unless you already know what they are trying to explain. The writing has to be about a topic for which the evaluator and the writer have a common level of understanding. This means that post-modern literary analysis is useless for teaching writing. Everyone can interpret the text any way they want, and each individual is expected to interpret it slightly differently. The correctness of the content is subjective. If the correctness of the content is objective, and mutually understood, then any confusion added during the writing is bad writing. Driving the excess confusion and length to zero is basically the problem of learning to write well.
In practice, writing isn't used to convey mutually understood ideas, it's used to convey novel ideas. But when evaluating writing that conveys a novel idea, the quality of the idea is mixed in with the quality of the writing. That's good for teaching how to think, but it just adds another variable in the way of teaching how to write.