Raise your voice, print some facts, present it to them and let them know they can't interupt you while you make your speech. Say, "I'm mad as hell, and I'm not going to take it anymore!" Nah, I'm kidding, but really, tell them, "hey! I listened to you, now you listen to me", but be serious about it. Don't be like, "mommy, I'm sorry, don't be mad." You're a big boy, you're 19.

If that fails, time to look for some PT work and move in with a few buddies. Go to school, move in with 3 or 4 friends in a place. The more people, the cheaper the rent. I know a few people who work only 20hrs a week, who live with 5 other people mind you, but get by just fine. You're 19 now and they shouldn't be surprised that their 19yr old is smoking pot. Surely they saw this coming.