Next time (if there is a next time) don't smoke it somewhere near your parents. Just tell her the truth and try to convince her you aren't going to anymore. Now get your friends to get the weed, and smoke it somewhere away from adults/police/snitches.
The following user thanked Sk8erFerSur for this useful post:
Yeah dude... Just tell her the truth. It's about your only option really. And then either stop smoking for a while until it's all in the clear or just be damn careful not to get caught.
Just tell her everyone does it, say you only did it once because it was peer pressure and you didn't like it. Say if you report me to the police or anyone all they'll do is take me away to a juvenile place for like a year.