Oh, I definitely think grammar needs to be a part of the rubric!! Face it, there are some people (like me LOL) who will overlook a story if it's terribly written in terms of grammar and mechanics, whereas it might be a very good story, content wise, if it was proofread. Writers, no matter what age, need to know that it's important, and I do think stories should be graded down if they are full of bad grammar. That's an area of improvement just as things like character development and continuity are.
And yeah, definitely, only the author can submit their story for review LOL.