Take deep breaths, maybe try meditation.
Get some water
Study prior to the exam
Use coping mechanisms that work for you such as a a stress ball or a fidget
Ask questions about the exam so you know how to prepare
Ex. What kind of questions will be on the test?
Maybe the teacher will say fill in the blank or multiple choice
Be comfortable with the material ask teachers for help
Sorry if none of this works, these are just some things that help me before a exam.