What should i do about my best friend?

well me and this other girl witch is my best friend are/wre friends with this girl sarah and we bullied us and over powed us just begin really nasty but before she did this to eather of us we became really good friends with us so the teachers woudnt really belive us if we said somin about her. Well this she started bulling us really bad it got to a point that we both wanted to move schools but we didnt becuase she was leaving to go to diffrent school wih her mams boyfriend or somthing like that dunoo really why they moved but anyway she has come back to our school again and has only been back for 2 days and it has started again just to let you know before she did anything to me it was my best friend and her ganaging up on me but then sarah started being mean to my best friend and sarah keeps trying to turn my best friend against me we told the teacher but all she said was she will keep a eye on things but she said it like she didnt care about it by the way sarah is the teachers pet everyone knows that so what should i do im really scared and my name is this because i dont want any of my friends or family to see this and think it is me so dont take any notice about my name sorry xxx HELP ME PLZ !