You are not allowed to break your fast because of exams. In fact, many find it easy to concentrate while fasting. So, perhaps you should try to visualize the spiritual benefits of fasting and turn to Allah to give you the stamina and strength. You may be able to endure rigors of fasting and thus succeed in giving exams.

Having said this, however, let me also point out that if you find yourself exhausted and dehydrated due to long hours of fasting, you are allowed to break it and make up for the day later.