if you got 72 hours before the big test, you gotta do this all 3 days:
drink shittons of orange juice, cranberry juice, green tea, coffee, whatever you like, and piss as much as possible. Then chow down on carbs. mashed potatoes, pancakes with syrup, ice cream shit hat will make you fat. Pizza is your friend. drink metamucil, and drop a few laxatives.
piss and shit as much as humanly possible. try to add a pound or two. DONT EXERCISE.
all the drinking and pissing will clear out your kidneys and urinary tract, and if you start gaining weight, your fat cells will be locked up, thus not releasing nasty old THC and whatnot into your blood. Shitting a lot will remove more dead cells and junk from your system than anything short of jacking up your head and running a new body under it.
3 days of that and you might even get your virginity back, just in time for the man to FUCK YOU by switching to a hair test (thats why i shave my dome)