Apparently, the child making the threats is a well known bully with a history of similar behavior. Can she sue the school?
Edit: she is contacting the police and threatening to contact the media. I suggested she retain a lawyer. I'll circle back with updates as this unfolds.
If the bully took action and the school did not, she would likely have a case.
Personally, I would not take a chance to leave my child up to chance of being harmed. Do what many of us already have, move to a private/charter school that represents your beliefs, or homeschool. Fuck the state indoctrination system.