You must understand that I'm not posting things that are not true. I have been advised via an organisation that will in due course be officially involved that had my DH tried to take her from the school or anywhere else for that matter than he would be arrested for 'taking someone against their will' father or not. If she was younger it would have been deemed acceptable... I think.
OP, it's not possible for people to give you support and advice while you are so ambiguous in your posts. You may not be posting untruths, but you are not explaining the situation, and the omissions are critical to subsequent readers understanding and learning from your post.
A teen reading this post would believe that they could leave their family home in favour of life with an unrelated adult, and their parents could do nothing about it. A predatory sexual groomer reading this post will believe that all they need do is pursuade a victim to refuse to return to her parents and there will be nothing the authorities can do, and a parent who has had their teen taken from them will believe that they are powerless.
Whatever "organisation" is advising you is not referring to uk law - no one can be arrested for "taking someone against their will" - there is no such offence. Someone has made that up.
The legislation regarding kidnap, false imprisonment, and abduction is very clear, there are no ambiguities. The age of a child is immaterial to the laws that apply in the UK - parental responsibility extends until the DC is 18, and is frequently enforced up to that age.
It's unlikely to the point of being ludicrous that a loving, safe and authorised father would be arrested for removing his daughter from the care of an abusive and violent man - not so ludicrous that the same father would face criticism, condemnation and prosecution for leaving his Daughter in the care of an abuser in order to avoid upsetting her. Many child neglect cases stem from a desire not to "upset" the DC. It's a fundamental part of parenting.
What is most telling is your DPs apparent unwillingness to act. If the situation is as you have described, without any other significant elements, then I know of no parent who would passively take the advice of "organisations" to leave his Daughter at risk in the hands of a violent man. Parents I know would argue, fight and demand help from anyone and everyone they could until someone listened. Parents have campaigned publicly against "the system" when their DCs face far less risk. Why has your DP not contacted the media? What about local MPs, the Chief police officer, head of children's services? If what you have said is true, then these people are ultimately responsible for allowing his DD to be abused.
Accepting the schools position, despite it clearly being against the law, ignoring his responsibilities in law in favour of a "wait and see" approach is reprehensible - unless, of course, there are good reasons for his reluctance to act, and the advice he has received is specific to circumstances that you have not shared on here, be that immigration issues, past criminality or medical factors, to name a few possibilities.