The cops should be called every time this other child threatens your kid or gets physical with her. Then, obtain every single police report. Also, get a local lawyer to send this other kid's parents a legal letter. As long as she is a minor, they are responsible for her actions if they know, or should know (if circumstances exist that would put a reasonable parent on notice)--the legal letter would put them on notice. That might get them to reign in their child. After that, you can sue the parents for civil assault (which is putting someone in imminent apprehension of harm), battery (hitting) and whatever cause of action would be against this other child. If you can show the court a pattern and course of harmful conduct, and that your daughter will suffer immediate and irreparable harm, you can also get a restraining order, at least a temporary one--that usually scares the school because then they have to keep them apart and monitor it.
As for the school, you need to obtain the school/district's written policy on bullying. They are obligated to follow it and investigate. But if their investigation does not reveal anything actionable in their opinion, they are not obligated to act further.
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