“Michael, you’re not about to do that Ravenclaw thing where you reverse engineer charms in your head, are you? I don’t mind really, but could you do it as we go?”

Haha! :D

Susan screwed up her face at that. “Interrupting cow, wh-?”
“Moo!”

INTERRUPTING COW! ♥ That joke is so ridiculous, it always makes me laugh.

This is really great. I love how you've characterized everyone, but Susan especially - giving her a physical disability is something that most authors don't do, and it's refreshing to see her so human. And she's still Susan, too.