What Would You Do? Really Confused!?

My close friend died in Feburary aged just 14. This week, a girl in the year below me started at my school and was telling lies about my friend. I knew her before she started at my school and never liked her anyway due to her bullying my best friend's little sister. She began to say that she was best friends with (Lets call her Katie) and that she was with her when she died, that they were best friends and she swore on her grave that it was her that called the ambulance when the car accient happened. Now this angered me to the extent that made me want to kill her. I am just learning to deal with her death and now the situation has hit the surface again it's making me miss her more and more and I can't grieve for her properly because whenever I think of her I think of that liar (let's call her Leanne) I see her in school, and because of what she said, many others in the school hate her too, which led her on to reporting us to the school. I got pulled out of class and got screamed at which made me even angrier! I don't even know why I got into trouble because I hadn't even had the chance to speak to her. I really really really want to give her a piece of my mind, especially how I told everyone that I would and don't want to be a liar like Leanne, but my mum works in my school and if I did that it would get reported and put shame on her. Also I am not a bully. She just can't get away with this! I know I won't be able to speak to her civily either because of my bad temper. Help, please? x