My eldest did regularly assault his TA (he is autistic).
I expected them to restrain him. There is no reason on earth why they should allow him to hit them, headbut them, bite them, etc without defending themselves (blocking, restraining, etc)- but - they know their job is 1:1 support of an autistic child who is aggressive. That's what they signed up for. I expected them to deal with the behaviour professionally while protecting themselves from physical injury.
Your situation is a little different in that that's NOT what you signed up for. There is no reason on earth why you should simply allow a child to attack you without protecting yourself.
Your problem here is management and lack of parental support.
What to do depends on how much you need the job. You could quit, you could refuse to work with her, you could deliver her directly to the head and walk away, you could get the union involved.
That girl is being totally failed. One day she's going to attack the wrong person and she's going to get the shock of her life and the shit kicked out of her. And that could be prevented if the parents and the school (management, I know you're doing what you can, with the restrictions placed upon you) dealt with the behaviour now. When she gets to secondary, and she thinks she can kick out at other kids - they're going to knock her teeth out.
I take no pleasure in saying that, I am not saying it's justice or good or deserved - because she's as much a victim of their failure to address the behaviour as everyone else is. I'm just saying that it's inevitable. She's going to lash out, in the end, at someone who will give it back.
(btw- my aggressive little boy, with the support and encouragement of everyone involved-us, school, etc - grew into a (mostly
) sweet and gentle young chap!)