Your DD is nearing the age where she can be transitioned to assisted living soon and I would be looking into this as soon as possible so you have the info.
In regard to her behaviour, although some of it is out of control with SEN kids, they do realise in a way, that the behaviour is also wrong.
You need to build a case now where you can prove she needs to be rehomed out of the home without turning in your parental rights, e.g. assisted living.
When she becomes violent, as difficult as it is, you need to call the police. Every time. You tell the police that your DD is out of control, attacked you and you're fearful she will do worse. There may be no action taken, but it will be on record how many times you have called, nature of incidents etc and these will be passed to SS also.
It is difficult to get a section 20 unless you have this proof to present to them and the support/testimony of attending officers at the time of the incidents.
Document all injuries she inflicts on you, bruises etc. Make a log book of all incidents where she hits herself (punching doors etc) as this will coincide with self inflicted injuries she may try to blame on you.
If this isnt nipped in the bud now, this will get worse when she is an adult. Make the hard decisions now and start documenting everything so you can present your side.