Start doing coke, fuck up your life real bad, then when you hit rock bottom say "You told me not to smoke weed so I had to do other shit that I wouldn't be caught for easily"

But really, maybe just be like "Its harmless..." then continue on with why it is. Tell her you use it responsibly, etc.

She already knows, shes suspicious, the most you can do, if ya' wanna continue tokin', is get her to care less about it. She may not fully believe you, but it helps to have your life on track. Don't mess up in school, don't get in trouble, appear productive, don't give in to any negative stoner-stereotype and slowly she'll realize its not as bad as it is.

Then you can live a bit under the radar, maybe you'll still get caught, but the punishment is sure to be less.

Then again, I don't know if honesty is the best policy...thats just one way of doing it. But like you said, shes already suspicious, so if your already getting caught, try that.