Making me look horrible

I’m in 6th grade and my best friend has left me and is acting like a bully along with another friend. She is now going out with my old boyfriend, telling people I’m the one who spoke bad things about her and others and convinced people that I had said I wish her dead. I get glares and dirty looks daily. I’m being ignored and isolated. I told the school and they said just keep ignoring it. If it gets worse let them know…
Why do people have to make an end of a friendship like world war? Doesn’t have to be like this. It’s so painful. My Mom and Dad are so upset too.