After hours of researching MG and YA literature on the net, a fervid email conversation on the SCBWI-Carolinas listserv with some helpful advice from YA author Alan Gratz, I realized it came down to this question:
Why are my characters the age they are now?
The answer has to be more than "because" no matter what I decide to do.
If it's an MG, then the answer MUST be MORE than
- because they aren't having sex
- because it's easier to write it that way
- because the action happens within a school
- because there's magic
- because there is sex
- because they are 15 years old (age is nothing more than a number in writing--they have to act their age--they have to be 15, not just be 15 because I say they are)
- because they are in high school (again, as an author, I have to make the high school a character, much like how NY is a character in Sex in the City)
- the lessons they learn in class are above the average middle schooler's head
- the ultimate battle they must fight is very dark, dealing with themes of temptation and redemption that might not be entirely appropriate for mid-grade
- the magic is school-based, which is a trope of mid-grade novels
- the story is focused more on self than on self's role in the world, which is a trope of mid-grade novels
- there is no romance, thoughts of romance, sex, or even infatuation in my novel. Anywhere. The characters are more focused on themselves and their discoveries than on the opposite sex and what can be discovered under their pants.
I think aging down the characters would be easy--but aging down the novel would be difficult. There are dark themes involved, and issues that I'd think would be easier for a high schooler to understand and identify with than a middle schooler.
I think making the tone of the novel closer to YA would be easy--but I can't lose the tropes that are more typical in MG (i.e. magic in school, witchy teacher). I can easily add in more realistic details to make the characters seem more their age, but the tropes of MG are essential to this novel.
The question still arises: whether to age-down my characters, or age-up my manuscript. There is no clear answer here, just a choice that I'm going to have to do one or the other.