Any help in a way to apologize?

Okay, there is this girl who is extremely important to me (She is my sister, mentally but not by blood). We have both had a questionable past and hers has left her really paranoid about being grabbed and such. Well I, in a stupid situation, did just that. And while we still laughed and stuff for the rest of the period (We are in high school) I have the feeling it was for the sake of the rest of the class. It unnerves some of them to see us fight. I need a way to really apologize. Because honestly, I think she is the only one keeping me sane, as she is the only one I truly trust. The feeling is mutual from her standpoint as well, if that helps in finding an answer for me. Any help or advice is TREMENDOUSLY appreciated.