Stress is exhausting. It takes it's toll both physically and emotionally. The intense concentration required during exams is also tiring. Easier said than done I know, but try to give yourself a break. Get plenty of rest, eat nourishing foods (preferably with some orange juice or other form of vitamin C to aid iron absorption) and be sure to drink enough water. Maybe start taking a multivitamin or a tonic like Floradix/Metatone just to boost you.
Firstly, I would get any medical issues ruled out. Have you had your iron levels and thyroid function checked for example? Maybe go to your GP and ask for a full blood test so that you can at least eliminate or treat any physical cause for the tiredness.
Are you up to date with sight tests too? Eye strain alone or a change in prescription can also cause fatigue.