Girl keeps smacking me?

ok well there is this girl that i know at my school.. we never really talk but recently shes being a ***** and very strange ANY time she sees me in the hall way she comes up to me and smacks me and i realllly dont understand why. ITs not a playfull smack either she does it as hard as she can and it hurts like a *****... so how do i get her to stop...(dont say hit her back) and i cant tell the teachers because our school is retarded and dosent care (were 16