I have a couple of principles we try to follow based on what has worked with my DS without knowing exactly what is going on on your side.
Don't let the behavior become the issue. The behavior is a symptom. Help her believe that you understand the issue and are working on solving it.
Try to stay calm, escalating your own reactions will feed the fire and give the signal that it is OK to escalate. The modeled behavior is often stronger than the intended message.
Beware of anxiety loops where their emotion creates a secondary emotional reaction and it spirals. Sorta like, "I don't want to go to school. I feel bad that I don't want to go to school. I really don't want to go to school because it makes me feel awful..."
We've found laughter to be the best way to catch DS before he goes full meltdown. Gives him the chance to erase some of the building emotions. That usually gives an opening to discuss things a bit.
