Your friends are idiots. If you ingest Comet, YOU WILL GET EXTREMELY SICK. It's not only about flushing out your urine. Marijuana gets into your blood stream and absorbed into the body. No amount of Comet is gonna clean that out and if anything, a whole new strange toxin is going to show up and it'll show the person testing you that you were TRYING to clean out your system. I'm not sure who's testing you or why, but I guarantee that these testers know all the tricks in the books. Marijuana will not be cleaned out of your system in a matter of a couple of days. If you smoke on rare occasions, your system MAY be clean within a week. If you're a frequent smoker, it can take over 10 and even up to 25-30 days. Eating comet will cause you to become extremely ill and you'll end up calling poison control before you die, then in the Emergency room drinking charcoal and or having your stomach pumped. I can tell you now, it's not fun. I bet next time you're constipated they say you should drink Draino! Before you ask that question, it's wrong too.

Your best chance is drinking EXTREME amounts of water from now until your exam and urinating as often as possible. Eating foods rich in protein like red meat will slightly increase creatinine levels because when you drink a ton of water your urine will seem diluted and you'll have less creatinine than a normal person should. Buy a detox tea from any drug store/pharmacy that has vitamin B12 in it since it will cause your urine to look more yellow even though it will be watered down from drinking too much water. Last but not least, you'll need to take a few aspirins. 3 or 4 aspirins will interfere with the test as well. None of this is an exact science because every body is different but for God's sake, even alice in wonderland knew not to drink bottles marked POISON!