Not that luck should really come into it. Certainly it is no substitute for a planned revision programme. It is really important in the final run up to the examinations to use the time wisely and follow all the advice you have been given. Going into the exam room fresh makes you able to think quickly and not make those silly little mistakes like misreading the question that can be so costly in terms of marks. Eating the right food can help to maximise your performance.
Research shows that drinking enough water is particularly important.
The brain does not function as well when the body is dehydrated. The examination period can be a very stressful time. Parents will need to try and be as supportive and sympathetic as possible. See pictures from the 2001 year 13 leavers assembly here. I have written separately to parents to explain the serious financial situation that Wiltshire Secondary Schools, including ourselves, are facing.