What is the name of this damn movie i cant sleep if I don’t find out

So basically a new girl who’s so pretty comes to a new school and then she becomes friends with a group of students and I think the first thing she said to them was something about cigarettes, and then they all start talking and then become friends. One of the girls in the group becomes her best friend. They always walk from school to her house. The new girl has great social skills too. One night the girl follows the new girl, and she find out that the new girl is dealing with a insane alcoholic or druggie mom (ion remember), the next day at school the new girl talks about her mom and how everything is fine and then she confronts the new girl infront of all her friends. Then the new girl gets so mad and after that she starts being so toxic and always plays the victim. Ion really remember what happens next but at the end the girl kills the new girl by a pillow.