What do I do if I'm being beaten up in school?

There is this girl in my gym cl who, along with a few other girls, I jokingly talk BS to. It's a friendly sort of thing and we do it back and forth. This girl, I'll call her Kay, on Monday started talking about whether or not I would hit her if she hit me. I answered yes and she said we would fight on Tuesday. Well, Tuesday I got sick and completely forgot about the encounter. When I came back on Wednesday she asked me why I chickened out. I was confused but regardless she began hitting me a couple times. I didn't react for I found no reason to do so. When I didn't react and walked away she tried to drag me to the ground, and failed because admittedly I'm not the skinniest girl in school. She tripped me and I got up and walked away and that was done. She seemed like it was just one of those BSing times by shaking my hand and apologizing. I was doubtful and lo and behold I come to school on Thursday and she beats on me in the locker room telling me if I don't hit back she'll keep doing it. I still don't retaliate and she stops to get dressed. I leave. Then when we go back to change again she starts it up again. I ask what happens if I hit her back and she says she'll hit me. I don't want to get suspended for the inevitable fight so I don't fight back. I feel it's to trivial to talk to the teachers about and they really don't do anything anyways, plus it's my own fault. I know this can be taken care of in a kind way without outside interference but I'm not sure how to go about it besides not fighting back but I fear by doing that I'll get seriously hurt because her attacks have become just slightly more harmful. What should I do?