Expelled seems an exaggeration. Suspended, sure. The girl who made the rude remark would probably get in trouble, too.
I support Smith for defending his wife, but I also support schools for enforcing rules. Smith is lucky he wasn't arrested or beaten up himself. That's what often happens when you hit another person in the grown-up world, which is why schools teach children not to do it, and reinforce that teaching with consequences. In real life, a person doing this at a work-related event would probably also get fired. Will Smith can't exactly get fired, but most people won't grow up to be so wealthy and powerful.
Life isn't fair, no one ever said it was.