I am an adult.
I have NEVER said to someone that "I am going to kill you" or "I am going to shoot you". Neither my DD(teenager) or any of her friends have threatened to do so. Neither has my DH or my mother or neices or nephews or my siblings. This verbal harassment and behavior might be based only on what a child has seen or what a child has heard and *might* not mean that a child (or even an adult) would actually *harm* someone, but that doesn't mean that I can or should condone it. I won't take that chance. If I EVER hear about another person threatening to kill any member of my family, charges will be filed with the appropriate authorities so fast the perpetrator's head will spin.
I don't care *what* age the perpetrator is, this type of harassment is a symptom of a disturbed psyche and of other anti-social tendencies. I mean, think of it this way...if a fellow human being (child or adult) is willing to say things like this in public, what do they do when no-one else is around? Pull wings off flies? Hurt pets? Physically abuse people weaker than themselves?
Plan a violent future attack on their classmates, on anybody who has supposedly wronged them?
To the OP:

Best of luck with this situation, whatever you decide to do. The things that many kids have to go through these days fill me with grief for all of us.
agnes!