After writing the man behind the commission, I was given a fresh perspective. Why not set it outside? Autumn can be walking to school along the sidewalk and stumble upon Dionne just as easily as she could do it walking to the classroom. (Plus, I don't fall into the trap of setting lockers in an elementary school.) I decided to try the venue shift, moving it to outside of the Poppinstock Academy.
This is obviously a very rough beginning. It has a loooong way to go before it looks right, but at least the underpainting is done. I'm not even sure how I'm going to actually treat the stuff yet - but that's a problem for the future. I hope it works out.