Try this: tell her you're going to come home randomly one night out of the next month blazed off your ass. And once drunk off your ass. If she even notices when you come home high, it'll be because you down a bag of chips and laugh extra hard at a comedy or something. Come home drunk, piss on the couch, take off your shirt and hang it on a plant, kiss the door and pass out in the hallway. "Hey mom, a bunch of people are out getting sloshed right now and all I'm doing is smiling and giggling. Is this really harmful?"

And you said it would make your parents sadder because you "want to smoke"... why the fuck do you think they think you smoked in the first place? If they *actually* think it's some demon that controls your actions they need to step into the twentyfirst century. It's a choice. The same way alcohol is a choice (one you're hopefully not making *shrug* personal preference)

Meh just don't let them know you smoke be stealthier about it.