Big NO according to my Bio teacher.... my doctor says they're no good to have before exams either.
You're much better off having something with a low GI, like a banana or an apple, right before the exam. Have a good drink of water too, but not so much that you need to take a toilet break during the exam.
Personally, Vitamin B12 saved me since the end of term 2. I've been taking an oral vitamin b12 spray every morning... which has really boosted my energy, taken me out of that sluggish mood and improved lethargy. Search for Pure Advantage B12 Spray in google..