It's probably not good to smoke weed as an addolescent though, I'd say she's right about that although I also think she's overreacting. I smoked pot daily (and did a lot of drugs) at your age, all parents should expect their kids to experiment with drugs at some point, generally in the teen years, and pot is definitly one of the less harmful ones for the developing brain.
Also, all the studies I've read that talked about damage to the developing brain due to cannabis use were studies about heavy smokers, if you only smoke once every two months you aren't doing anything that could damage your brain as far as I know.
If your mom is ok with alcohol or tobacco you could point out that both of them are far, far more risky then marijuana. It's pretty easy to find pro-marijuana stuff online, but she might be more convinced by unbiased info. Sources like erowid are great and they're fairly unbiased as far as the information they provide. If you read the information about marijuana though, then you get the picture of weed being fairly harmless, especially with such occaisional use, because it is. You don't need a biased source to tell you that pot's a safe drug.
