["Seems" would be the key word here. While Marcia is indeed engrossed in her studies, much as she has been ever since prom started, she's still observant as ever. So when Rin decides to try and swipe one of her books, a hand shoots out and quickly yanks the book back.]

It would have been more polite to ask, you know.

[She fixes a firm stare on the girl.]

Are you incapable of getting your own books, or do you just enjoy stealing?