Karen is informed by owl that she's a prefect. She writes her friends about that, of course.
"Uh, I can give you a message and you can convey it if you think it will be helpful? Karen said, and I really can't imagine caring."
"If you're secretly half dragon and half merperson and Transfigured into human form by a dark wizard who will release the curse if you ever go on a date with me then I guess your parentage would be relevant."
"Right, where could I possibly have got the idea that you care who people's grandparents are."
"I'm pretty sure. Unless your dragon ancestor is the same one as mine, then all bets are off - Antipodean Opaleye?"
"Ah, no, our secret dragon ancestor is a Hebridian Black, we're all right after all."
"So what'd Karen say, I haven't got Cricket spelling yet so he has limited vocabulary."
"She said you weren't a pureblood and thought I might care, and I said I hadn't even realized you were pretending to be. As opposed to just counting on it being rude to say you weren't."
"I don't like lying and nobody's backed me into enough of a corner or guessed close enough that I had to." Sigh. "Timothy probably guesses but hasn't said anything."