Is wrongly accusing someone of underage drinking slander? What should I say to the person?

Basically, I'm 18, a manipulative girls mom on my old swim team saw pictures of me and the older girls at a club for my birthday on facebook. None with alcohol in the pictures. I did NOT drink. The girls mom pulls crap like this all the time, talking about people behind their backs and making them look bad, and I've had enough. I care about the respect of my old coach, and she complained to him that "hope holly mary and courtney go out and get drunk every weekend and i see it on facebook" (im courtney) now.. this pissed me off cause this woman has never spoken more than small talk to me, and if she thinks she can get away with making me look bad out of bitterness, she's wrong. I know she hates face to face confrontation cause she can't hid. So tomorrow I'm going to the Ymca and talking to her, face to face, and calmly telling her its not right to slander someone like that. What should I say?