Has this ever happened to you?

You're riding on a bus. Yes, a bus. It's a crowded bus because a course in the afternoon just ended and everybody is of course going home. You're sitting down in one of the seats, and then a woman decides to sit on you. Now, you know this woman, she's in your Psychology class. You're not exactly friends, but you do know each other and have had conversation. What is the social norm when a woman decides to use you as a seat on a crowded bus?