In the old days, I used to proudly announce that I don't believe in grading creative work. It was part of my training. I went to schools where whatever I turned in to a CW class was given an A. And I can assure you, it wasn't because all my work was excellent.
Over time, however, I've come to see that treating students as if they are all young geniuses who might be harmed by being corrected or edited is silly and maybe even lazy. In fact, such teaching isn't teaching at all but a great disservice. There is plenty to learn about writing, and plenty to call "mistakes" -- even though the mistakes might become something quite powerful if you really push them. But first you have to know what you're up to.
This is why we draft and workshop. We draft, change, discuss and even argue about fiction in the classroom. (It's always good when there is real disagreement, as there was on Friday.) And I insist on small group workshops as you develop your stories. Mostly I am guiding you toward an understanding of fictional strategies--your own, your classmates', Ernest Hemingway's, and Selah Saterstrom's. Because I work on the theory that all writing is revision, I want you to be able to see what you do as mechanical even as we all know that good writing is inspired and can't really be quantified. Here, we have to quantify it. And I have to give grades.
So for your stories I give one grade for the story and one for the mechanics--your grammar and punctuation. (Those quotations!) They are averaged together for your total grade, so poor punctuation can cost you. The first three stories won't count as much as the two you work on for the portfolio, but super low grades can be a problem. That's why I allow revisions to improve the grade.