Before you give blood, drink lots of water. Everyone kept telling me that would have helped. And don't drink too much coffee, they said caffeine can dehydrate you. Of course, by that time I found all that out it was much too late.

invigilators

I've never heard that word before, but I'm so going to call myself that now. It's perfect. I had this vision of a Dementor kind of person in a robe, ready to suck your soul out if you cheat. :)