Can i say what i think of someone online without getting in trouble?

This girl is creating rumours about me since i'm going out with her ex boyfriend. And i want to post something on her wall to her. I've read before that if you don't say for example someone is a *****, but you say that in your opinion she is, you can't get in trouble. I need ways to get around this slander law and stuff. Because she has before and will again take things to the deans or principals. Thanks!