How can I get this girl to stop making fun of my ?

I am 17 years old, and I have a tiny (3 inches erect), and this really hot 16 year old kinda slutty chick who I hardly know saw my one time, when she walked past our locker-room, and the door was open. And now she comes up to me almost every day, puts her hand down my waistband, grabs my , says "still no growth" laughs, and walks to her friends who is dying of laughter, even when I'm around friends, and sometimes when I sit, she just randomly comes over to me and sits down on my lap, or makes me grab her or . To be honest I really enjoy it, but I know she does it just to humiliate me, and I have no idea how to make her stop.