Good diet is ESSENTIAL to a focused mind, try eating small meals 4-5 times a day which should include fresh fruit and veg. Never miss your breakfast, drink a minimum of 2 litres of water a day and you will be amazed at the changes!
Also I was told to take a herbal supplement from my nutitionist called Rhodiola you can buy it in any good health shop. It has quite an impact on concentration and general awareness. I was not so sure but I took it and after 2-3 weeks the affects were dramatic. It's best if you get the root part of Rhodiola as that's the more concentrated part.
Also try 30-45 mins of general relaxation/meditation a day which should be easy to do.
Finally follow all this up with a good nights sleep and you will be flying through your exams
Best of luck!