"Mom, I realize that I am underage and that I am living under your roof, so I will abide by your request that I not smoke cannabis while I am living in your home. That being said, I think that you should know the facts about cannabis, and make your own decision about it." (Then, present your facts.)

And then, don't smoke. Maybe you can ask permission to smoke on special occasions or events, or maybe your dad would let you when your mom is out of town. But I wouldn't go behind their backs, cause if you get caught again you will get in REAL trouble cause you broke a promise.


Here is a book that I highly recommend, and if your mom has a college level science vocabulary, I would have her read the chapter on cannabis:

http://www.amazon.com/Primer-Drug-Ac.../dp/080507158X