I think you are a bit misinformed.
Actually, the brain vitually runs exclusively on glucose, which is a simple carb. The only exception to this would be during intense exercise in which the brain may run on lactate or keytones to my knowledge. I think you are confusing cholesterol usage in the brain. Cholesterol plays an active and very important role in brain funtion, but the body makes enough cholesterol to support itself. In order for the fats to be used as fuel it would have to be broken down to a CHO structure and then burned as fuel.
Stick with a complex carb like oats, sweet potatoes, bran, or something like granola 1-2 hours before the exams. Good luck.