Was it wrong for my cousin to slap a girl and make her cry?

I am a guy, and I am not a fan of guys hitting girls. However, if your cousin's story is true, and she was really doing all of the things mentioned in the story, including pulling down his pants, I personally believe she deserved to be slapped. Hopefully a lot of people were around to witness the events, so that the girl can receive punishment as well. This being said, your cousin probably still deserves the suspension. In high school, violence as such cannot be tolerated, no matter the action that caused it.