Arvid holds a book when Mr. Moore asks him if he thinks he's a nerd. It then cuts to a close-up of Arvid, and his hands move around freely, indicating he isn't holding anything. In the next longer shots of him, he holds the book and then doesn't.
When Arvid comes back in to announce he's not going to the dance with Laurie, he walks across the room, still standing up. When Dr. Samuels comes to talk to him, he is suddenly sitting down.