Because I am a smart ass at times, in this situation I would google a bunch of famous cake decorators, print out pictures of their work, and leave them on said art teachers desk with a note that said ''What is art?''. No verbal communication, no glares, or 'humph' attitudes, just plain and simple.
It's definitely a form of art, according to my definition, which being: Anything created by hand that evokes a sort of emotion from not only the creator, but the observor.
I think that it might not be appropriate for a school art class because aren't they supposed to focus more on the visual, permenant arts? Food + schools never turns out nicely, it seems.