The brute fact is, the instructor in a fiction workshop earns his pay by telling students what's wrong with their stories. The students themselves are convinced they need encouragement more than anything, and of course you'll encourage them as much as you can; but what they need most of all is discouragement, so that they'll come to realize how appallingly low their standards are and break the terrible habits they've learned.
As I believe in passive sadism in childrearing, so I advocate the same stance in dealing with the obstreperous student. Kill him with kindness or at least benevolent inattention. Not only must you never let yourself be drawn into any sort of emotional escalation, you must avoid acknowledging his attitude.
Make sure you have something to say before you write it down. One of the most difficult things undergrads have to learn is they have as yet little to say.
Many nonfiction teachers make the dumb mistake of providing subjects or topics. Let the student choose them himself, and make damn sure he says something about the subject--rather than merely turning in a description or summary or noncommittal analysis of it.
For some cockeyed reason it is assumed that if you have the required degree you can therefore do an adequate job of teaching.
Often a classroom of students will unconsciously follow a peer leader--a sarcastic put-down artist, for instance, who by dint of personality and precocious verbal skills will turn your course into a living nightmare unless you step in and blandly damp him off.
It's quite true that fiction can't be taught; but you can pass along a few shortcuts and get them interested in the craft of it. I don't think any student wastes his time in a good fiction workshop, not even the talentless ones.
Undergrads tend to use more words than they need to, and much of your work involves showing them that a certain word or phrase or sentence or paragraph can be deleted without loss.
The most prevalent problem in student fiction writing is lack of plot or suspense, or drama.
Undergrad fiction writers are intensely interested in the possibilities of metaphor, simile, alliteration, allusion, parallelism, symbolism, and all the other literary devices. Which is fine. The problem is that they're more interested in the devices themselves than in using them effectively.
For student writers one of the most difficult problems is "creating character"--and it's a damned hard thing to teach.
Fiction-writing students would much rather describe than narrate. Would rather tell than show. Would rather summarize than dramatize. Would rather explain than demonstrate. Would rather obscure than clarify. I don't know why...but students seem to want to do everything wrong.
The amateur's attitude: It is I who am doing this thing, and I'm more important than the thing I am doing. The professional's attitude: This thing I'm doing is more important than me. (In other words, just because you wrote it doesn't make it good, or even interesting.)