It seems like you cannot take the pressure of the exam.
The simplest way to overcome this problem is to take more and more test so that you can become familiar with exam pattern and test taking.
Ask your parents or anyone in the family to take some time to prepare test for you and take time to attempt the questions thoroughly. The more you practice, the lesser you will face problems in the examination centre.
Moreover, also take a good sleep the night before the examination. You must not go to the exam centre empty stomach, take a healthy breakfast before going to the school and also drink water or liquids.
This will help you in keeping your mind fresh and healthy in the examination centre.