If the bullies are also 13 then the courts aren't going to do much therefore the "court of public opinion" is the right court.
Anybody who has successfully overcome bullying (as I have) knows that bullies only understand one language. And you need to speak this language eloquently, for them to understand that terrorizing other kids is unacceptable.
It's too late for this girl, but I hope for the sake of future victims that the family can come to learn the value of a solid alibi, no witnesses or cameras, evidence cleanup, and location obfuscation.