I don't smoke myself but a friend of mine is quite into it and went through something similar. Drink a shit ton of water, at least a liter every day until the day of the test, wouldn't hurt to try what Boom said the day before the test too. THC takes normally around a month to get out of your system, so just make sure you drink plenty of water each day to speed up the process. Make it subtle obviously, if your mom sees you drinking uncommon amounts of water from one day to another she will notice.
edit: btw do the water drinking throughout the day, taking in 1-2 gallons of water in a sitting and can make you feel really bad and might even kill you.
edit: btw do the water drinking throughout the day, taking in 1-2 gallons of water in a sitting and can make you feel really bad and might even kill you.