What Tootsie said was right on from a parental perspective. It just surprised her and shocked her and worried her. Moms react that way when it's their kids, and she's obviously responding through a lot of the propaganda that all our parents (and many of our contemporaries) believe about cannabis.

Try and give her a break. She's worried about your success in college and your motivation. And above all, any caring mom with sense is going to be worried about the fact that, whether cannabis is harmless or not, it's illegal and can result in your getting arrested. Nothing in the world is going to take that worry away from a mother, and I'm sure you can understand that.

I think you should begin a deliberate, gradual campaign of education and information about cannabis to help persuade her. Print out pertinent articles and studies from the NORML and Erowid sites. This is a battle cannabis advocates have to wage one person at a time, and there's no better person to start with than mom. Good luck!