I'm about old enough to be your granny and I work in education. Your Mom is worried about all the stuff she's hears over the years about how pot can mess up your life. You are going to have an uphill battle about pot with her. I just wanted to give you a little ammunition. There was a scientific studdy that showed that MODERATE pot use is good for your IQ! This is just the abstract (summary) of the study, but it puts it in no uncertain terms that smoking less that 5 joints a week raises your IQ the most! Smoking MORE than 5 joints a week, however is bad for your IQ! http://www.cmaj.ca/cgi/content/abstract/166/7/887
Nor does it mess up your hormones. http://www.ncbi.nlm.nih.gov/entrez/q...564&query_hl=1
Or mess up your memory forever (stoned, yeah, your memory's shot, but straight, it will still work fine) http://marijuana.researchtoday.net/archive/3/3664.htm
Or give you cancer like tobacco. (In fact, there is a lot of evidence that pot kills cancer cells- if you're interested, just google- cannabis abstract cancer)
http://www.harmreductionjournal.com/content/2/1/21

I would strongly suggest that you be honest with your mom and tell her that you will keep your use moderate, and not drive (or be driven by friends who are) stoned. That you will keep your grades up (a big thing!) and act like a responsible person. Keeping your act together can go a long way in convincing her. And for your health's sake, I strongly suggest a vaporizer (Vapor Brother's is a good one -about $150). You use 1/2 the pot, your lungs will stay clear and what's left over can go into brownies). Armed with these studies, I think you can argue your case for responsible use. Good luck!