how to tell someone to not talk to me for sometime (politely)

there is a senior from my school and we go home together by bus and while waiting for the bus she would always talk to me about stuff and i used to like talking to her but she is starting to get a bit rude. she slaps my arms and pinches me if i look somewhere else when i am talking to her. she used to go to my tuition by got kicked out cuz she got into a beef with another senior and instead of handling the situation among themselves she beat the shit out of the other senior's sister. she also bad mouthed my friend. idk what to do cuz o dont wanna have a fight with her too. she gets angry over small things and doesn't really care abt other's opinion so i dont know how to tell her stuff. pls advice.