I want to get her work the exposure it merits, but how do you draw the line between mentoring and co-authorship? At what point in my scrubbing and critiquing am I pushing 'our' work instead of 'hers?' Is co-authoring an appropriate form of mentorship? Should I let her grow by herself among the talented teens, or give her a running start into true professionalism?

I know that's long on questions and short on explanations, but that's all I got right now.

Sit down with her and critique it face-to-face. Also mark up the manuscript and give it to her. Then, let her make the decisions on what to change or not change. After all, she might be right and you wrong on some things. Advise her, but let her make her own mistakes and claim her own victories.

Nothing wrong with that, if it's your honest appraisal. And it's far from "co-authoring". If you want to put your names together on the byline, then you can move into the territory of writing parts of it yourself and sharing artistic control and all. I don't think there's necessarily anything wrong with that either, it just seems like a lot more work than I'd put into somebody else's stories.

When I critique, I usually stick to what I personally like or dislike, rather than what a given or hypothetical market might buy. When I feel that I and the intended market for a work would have a difference of opinion, I go ahead and specify that difference ("I like X, but [market] probably would prefer Y"). It's important to keep that sort of thing in mind, as a bar to getting overly controlling of another person's work. Some stories will have merit but not be your thing. And maybe there's value in her writing for teens.

The big danger here is that you'll introduce an element of pressure that will foreclose some of her own development as a writer. Or rather, as a person who also writes, rather than a humanoid text output device. If writing is the number one thing in her life, she'll never become capable of writing anything worth reading, only spitting out what she's been programmed to produce. The other danger is that she'll develop an allergy to writing and quit because it's such a hassle.

You probably hold this awareness, but I feel I should say it anyhow. Be acutely aware of the power differential between you. One of the things about being a teen is that they haven't figured power boundaries out yet... that is, your advice may come across as "do this" rather than "consider the possibility." If you are older, and in a professional role (teacher), then you are NOT peers, and should not interact as peers.

I'm also making an assumption you are male. Correct me if 'm wrong. Between a teenage girl, and an adult male, there is an entire social issue here as well, beyond that of the writing, and I think you are aware of it or else you wouldn't even have felt you needed to ask this question.

If it were me, I'd stick to discussing the "why" of it... give her your rationale for flagging a particular passage and say why it works or doesn't work... but do NOT rewrite it for her. Co-authoring is something to be done between peers. Instructing is about dissecting the mechanics of what works and what doesn't, and pointing a writer in the right direction.

Give her advice about how to access resources but don't push her, and don't do it for her. Once you step into the role of co-author, or manager, or publicist, you've crossed a power barrier between teen student and adult teacher that perhaps was not yours to cross.