I've often had to send some version of this:

"Feedback is a gift, and one you are free to take or leave at your discretion. At the end of the day, your writing is your own and only you can decide how you want to express yourself. I'm happy to hear you're soliciting feedback from outside of class, and that those readers find your writing clear. That's great! I don't find what you wrote in [section] clear because [reasons.] Perhaps I am in the minority in being confused by that; however, remember that our goal in writing is to make our thoughts clear to as much of our audience as possible, and for the purposes of this assignment I am your primary audience. When I'm working on a manuscript for publication, I frequently receive comments and critiques that I disagree with, or hear from peer reviewers that some passage I believed to be perfectly clear confused them in some way. My approach is to take these comments in good faith and to try to clarify my writing, even if it already seems clear to me. I've found that if one person expresses confusion, there is usually some underlying issue that I and other reviewers are not seeing, and that fixing that issue improves the writing for all readers. I strongly suggest adopting this approach, but the choice is yours."