I can sympathise with your distress, but would also request you very strongly, not to be so upset. I agree that the Principal should not have used such words to describe any child in her school. But sometimes, Heads of schools get harassed with the demand for admission and for special treatment by parents who flaunt their wealth and social status. It is quite likely that she has had some unpleasant encounters with such parents and unfortunately put you and your child in the same category.
Perhaps, if it is feasible, you should consider changing schools. If that is not possible, you should make your peace with the school authorities and get on with life.
Please find out if the child does have a learning difficulty. This condition is not permanent and can be helped, with the right kind of guidance. One of my friends calls it Teaching disability rather than learning disability. But this a joke to cheer you up, which is NOT to be shared with the school! I do not know where you live. If you have access to a specialist, you could get your child assessed. There may be a few simple procedures to following order to help her, if she needs it.
Most schools start reading and writing for the children too early. In the US, children are 6 plus when they join First Grade. Here we tend to start writing classes in the KG. But most of us are victims of the school system.
Children need to have friends of their own age and many of them attend school only for that reason. Nevertheless, most children survive their dull classes to make something of themselves. Their natural resilience and energy serve as their best resource. You should not get so angry that it affects your BP and sugar levels. Some exercises in Yoga could help you. Your own decision to stay calm and not to get upset will be the suggested treatment. All the best.