What do you guys say to your best friend who wants to have a affair? PLEASE HELP!?

my friend Rajita and I have been friends since about 2nd grade. We are in 10th grade now, and she recently professed to me that she wants to have with me. i thought she was joking around at first, but now i know that she is serious. one time she pushed me into an empty clroom after school and started touching me and licking me. it was a very nice gesture, but i am not . how can i give her the message without hurting her feelings? thanks. please help whenever i see her she keeps trying to touch my ***. its getting to the point where even my mother is noticing her behaviour. i just don't want to ruin our friendship, so i try not to say stop when she is doing these things. how can i tell her??? AHHH please help! thanks!