duuude same situation here. my parents know but its always awkward so i just try to cover it up anyway.. im sick too so i went and got nyquil at the store, and smoked on the way home. but if your out of a car, run to your friends to get the new season of whateverthefuck and say its for tomorrow so you can take it easy and get better
slip some acid in your moms drink,trust me it will work,she'll be tripping to hard to notice you going out to smoke,you could just kill her tho,that would probablly be a faster approach to your promblem,i suggest you stuff her in a wall like that yale student to get rid of evidence,or you could just say your going for a walk,i mean shit your 18 right.....................
Excuse or no excuse, you're going to come back in the house reeking marijuana blazed outta your skull. So you might as well get it over with; can't hide it from them forever. Either that or wait until they go to sleep.