I work for an agency that takes care of senior citizens in their homes. I have many women who go to a hairdresser weekly, get their hair washed and done and that is the only day they usually have their hair washed. The rest of the week, the basically brush or comb and go. And their hair is not dirty. Maybe not looking so great by the next time they go to the hairdresser, but for some reason, it doesn't appear dirty.
In this child's case, it sounds like the hair has not been washed in some time, let alone combed or brushed. Also, if the child smells that bad, that your eyes water, I cannot imagine when she was bathed last. It seems the dad is not so much neglectful, as clueless.
In this case, I would notify the school guidance counselor and let them know of this concern. If the child smells that badly, it will cause her harm in the way that other kids will not want to be around her. And I cannot believe that the teachers have not noticed. I worked with elementary kids and sooner or later, the teacher comes into close contact with their students.