In 2001 I worked at a juvenile prison for boys under 18. One of my tasks was to give every entering student a reading test; he would take the identical test when he was to be released as much as 3 years later.
Most delinquents have had educational problems so when I announced this test they would groan or become frightened. To each one I said, “I have a secret for taking tests. Studies have shown that caffeine in coffee or chocolate and sugar will improve your brain. I will give you a small candy bar (coffee was not allowed for prisoners) before the test and you can see how much better you do.”
I bought a bag of small candy bars, mostly Snickers, and kept them for this purpose. The results were amazing, so amazing that my supervisor bought a whole bag of bars for me. Every single student who took the test (except one) improved their grade. The prison officials were happy because it looked as though they were doing a bang-up job in teaching.
Why was this effective? First, it is true that caffeine, sugar and chocolate do affect the brain and so the student was really perked up. But second, feeling negative about tests has a bad impact on one’s abilities. Students who are repeatedly told "you are dumb, you can’t make it, you will fail," usually do. These boys went into the test believing they would do well, and they did. Having a positive attitude about tests, or any other task, improves your chances of success.