Honestly I would simply tell her that it is not fair for her to be held to a different standard than the other children (fair does not mean equal, but right for everyone). If she is expected to do different work then that work should be held to the same standards as the other children's work, but if they are not expected to redo messy work or redo wrong answers then she should not be expected to redo messy work or redo correct(!!!) answers.
The class is right for the other students, but is no longer right for her, therefore the best way to be fair to her is to move her to another class with more objective expectations. Be honest with her. Kids tend to have a sense of what is right and I'm sure she feels that something is off.
