I don't know if anyone else has had exams at the moment, but I thought I would share this with you anyway for future reference.
This is my essential kit for exam days. Take notes, there'll be a test later...
Scarf - They might make you take it off during a test, but this is essential, especially if they make you wait outside like they do at my school.
Umbrella - Again, this is an essential, at least for January exams in the UK. It would be horrible to have to sit through a 2 hour exam when soaking wet and cold
Chocolate - First of all, I do not advise a sugar high just before any exam, especially a long-haul one. You'll just come crashing down mid-test and be unable to work any longer
Tissues - Sometimes, your body picks the worst time to be ill. Be prepared, because sneezing on the person in front is not cool, no matter how much you dislike them.
Vaseline - I don't know about you, but my skin and particularly lips go very dry when I'm stressed, then I start biting my lip when deep in thought. maintain your lips and sanity with some kind of lip balm.
Diary/Planner - It simply wouldn't do to miss an examination and have people think you are unorganised!
Water - This is another thing they might take off you, but sometimes they'll keep it at the front and you can ask for some when you need it. Just don't drink too much, or you know what will happen...
Calculator - Depends what subject, but it's always good to keep one on hand
Pencil Case - make it look cool so that you have a shining beacon of hope to help you through to the end. For my ideas on how to personalise yours, check out my post on the matter.