My experience of taking exams was that having taken normal insulin and meal beforehand, my BS would rise 2 or 3 mml more than usual (which I always put down to stress). However, once the exam started and I began writing and thinking, it dropped steadily. By the end of a 3 hr paper, it would always be at the lower end of the normal range and I would need something to eat. It's generally acknowledged that stress sends your BS levels up, like anything else that may cause anxiety, such as any sort of medical procedure in hospital, some tests, surgery etc. As soon as the cause of the stress is over, it is likely to drop again.
Good luck with your exams.