Reminds me of a friend in high school -- she turned in an English paper with something unrelated in middle. I can't remember if it was song lyrics copied out, or rambling on about something, but the teacher didn't even notice because of the previous good work she'd done.

But I also like the way you worked their characters into what might be a typical high school occurrence -- Lily's insistence on fairness and Snape mocking her Gryffindor sensibilities. Nice fic, and very plausible.