You as an adult have the ability to modulate your voice, to be intentional in the way you are going to address children. It is important that you do not forget to focus on this process and do not despair. It doesn't matter if the screaming continues, you keep the focus on modulating your voice. They will learn!
Remember that they are learning to modulate their voice and you are being their example. The voice will be one of your best allies so that children can learn anything you present to them: addition, learning to read, learning to love, learning to express themselves, etc.
Have you tried speaking very quietly in the middle of a noisy classroom? If you haven't, try it, you will see the difference.