Your dd sounds lovely
I'm so sorry you both have to go through this.
Your dd obviously is a very kind little girl if she does things like take him to the medical room, so I would talk to her and explain that he probably has some problems that make him angry just so that she doesn't start to feel that it's her fault. However, you also don't want her to feel so sorry for him that she lets him get away with being horrible to her, so I would think about trying to teach her ways to recognise when he is getting angry or is about to lash out so that she knows when to get help from an adult or to simply move away from him. Giving her some words to use when he does something that she doesn't like might help too, and if she is feeling some kindness towards this boy you could tell her that it will actually help him to hear these things from her so that she doesn't then feel like she is being mean to the boy with problems.
That could all be useless advice, or something that you are doing already, but I think in a situation like this, anthing you can do to help yourself and your poor dd to feel that little bit more empowered has got to be a good thing.
You could even talk to the teacher about it and ask them what strategies they are putting into place to help manage this behaviour in the classroom, so that you can reinforce it at home. This might also have the benefit of showing the school that you are not going to let this lie, but you do have some understanding of the difficulty they are facing as well.
I'd also think that if the Father just laughed and shrugged off his childs behaviour with 'that's what kids do', he really isn't that much of an involved parent who cares a great deal about their child's happiness. And lets face it, this boy can't be a particularly happy child if he is bahaving liek this on a regular basis. I'm sure the majority of parents would be mortified to know that their child was doing this, and if he had that attitude to you, he probably hasn't exactly been cooperative when the school have spoken to him either. Which could be one of the many reasons why the school thinks it would be easier to move your child rather than theirs. But if you feel strongly that you want your dd to saty in the same class, I would start putting forward the case why she should stay put now, as there isn't much of this year left. Think of all the reasons why it would be detrimental to your dd to be moved and present them to the head in a meeting.