I have a 5-page article on this coming out next year in the Journal of Legal Education, and I'd love to get people's feedback on it. Unfortunately, while the Journal is great about leaving authors with unlimited rights to redistribute the article after it's published, it's behind the law reviews in letting authors post prepublication drafts on blogs and on SSRN. But if you're interested in looking over this, just e-mail me at volokh at law.ucla.edu, and I'll gladly send you a draft. Here are some excerpts:

We often mark up student papers, and note errors in grammar, spelling, and word choice. But, unfortunately, much of what we know about such supposed "errors" is itself erroneous. [Examples omitted.] ...

Why is all this important? For four reasons:
1. Accuracy....
2. Protecting ourselves from embarrassment....
3. Protecting our students from embarrassment....
4. Helping students understand the nature of rules....

Here then are a few thoughts about how we can avoid such errors: ...
6. Focus on function and not on abstract correctness. Finally, as I suggested above, tell your students what's effective writing, rather than limiting yourself to what's correct and incorrect. By teaching them that even usages that are approved by the dictionary can still be distracting, alienating, clumsy, or otherwise ineffective, you can help your students think more broadly about how they can improve their work. And that, after all, should be both their goal and yours.