There were kids in my high school back in the 80s with hair like that. Several of them. No problems reported.
It would be one thing if her hair were bright pink, or shaped into 6” spikes. Threatening her over that hairstyle is beyond intolerant. Were I the parent, I’d inform the school that I expected to see every girl there cut their hair short as well, or they’d be hearing from a lawyer. Just because her hair doesn’t lay utterly flat (i.e. like a caucasian or asian’s would) is no reason to demand it be cut or else.
Or they could ban big hair altogether as a safety issue.
If you're attending a religious school, most of the time you are expected to dress and keep your appearance up to certain standards.
“There were kids in my high school back in the 80s with hair like that.”
Oh yeah, in fact way bigger ‘fros than that gal has. And those were just the Jewish guys.