How Does a Math Teacher Tell a Kid How to Write?

I know, it's Mathematical Monday and this is only loosely mathematical, but it's the question on my mind at the moment.

I like showing my students that people don't (and shouldn't) fit into neat little pigeonholes. I like encouraging them to be multifaceted and be their entire selves in my math classes. But there's a drawback. Kind of.

As my students find out about me being an author, a few will ask me to read something they wrote.

That's cool, in theory. At my last school, we were such a small, tight community that it wasn't really a problem. But now, I find myself unsure how to respond.

After I read it, what do I say?

"That's great! Keep at it."

"I like how you describe the forest. Just watch your run-on sentences."

"Great start. Here are some things you might consider to tighten the narration and give us a stronger point-of-view."

Do I just give general encouragement? A tip or two? Or deeper feedback? Sometimes I just don't know, because I'm not the English teacher.

If I were the English teacher, I'd know what kinds of things we'd already discussed in class. The things kids want to show me aren't always for class assignments (some are actually doing NaNoWriMo at the direction of our librarian—which is awesome). But when it is an assignment, maybe there's something specific they're focusing on. As the math teacher, I have no idea.

Perhaps I'm worrying too much. With so many students, it's hard for me to get to know individuals well enough to know what kind of feedback they want/are ready for. Not like my last school, where I had super-tiny classes and often taught specific students for up to four or five years.

If anyone out there is an English teacher, can you answer this question? If you had a novel-writing math teacher in your school, how could she best support your students' writing efforts, without undermining any methods you're using in class?