Don't worry!! I'm in yr 11 and i have the same fear!! I have anxiety and have broken down in many tests but the trick is (as well as revision which will assure you that you know the subject) Deep breathing. It really helped me to calm down in the exams that i sat (and i hope ti helps in the mocks i'm about to sit!) You could also ask your school if it could be arranged for you to sit in a separate room or at the back idk.
ALSO ALSO one of my teachers told me about this girl who was an amazing student who was predicted a 9 but got a 4 due to a panic attack in the middle of the exam. So my teacher wrote to her 6th form explaining that she is an amazing student who consistently does well (and she gave proof) and she was accepted! So don't worry should anything arise there is always something that can be done.
Hope this helps :^)))