How do I handle constant crises and competing demands at school?

You can run the operations now, which means the ordinary days no longer scare you. What buries you is the pile-up: the crisis on top of the competing demands on top of the work that was already full. That is not a sign you are not cut out for this. It is a sign that too much is landing on one person at once. Crises will not stop coming. That is the nature of a school full of people. So the skill is not preventing them but sorting them. When everything feels urgent, very little truly is. Ask which fire will actually cause harm if it waits an hour, and which just feels loud. Handle the harmful one, and let the loud ones hold. Most competing demands can, in fact, take a number. When the day floods, stop and name the one thing that must be handled now, and give yourself permission to let the rest wait in line. You cannot do all of it at once, and pretending you can is what buries you. Steady sorting, not heroics, is how you carry a job that never stops arriving.