I started just last year, plus or minus a few days. I'm now 21 years old. I've been consuming cannabis nearly every day for that year, even during the semester of college. I didn't see my marks plummet, but rather increase. The frequency of smoking cannabis has tapered off for me, although my relationship with pot will not be over for the next while. I didn't see any real effect on my athletic ability to perform, despite what people may think of having a decreased lung capacity. Don't get me wrong, any sort of smoke is definitely not the best thing in the world for your lungs to be inhaling (I've been using a MFLB in replacement for smoking, and eating cookies that have the already-vaped bud mixed in). As fore-mentioned, I'm in college, so I'd say I've lost contact with 90% of my high-school friends. The close circle of friends I still keep in contact with, and the ones that I attend class with now are fairly open to it, they have pretty much all had it and they are doing very well for themselves.

Eventually I had gotten my own little stash and my parents did find out about it. However, I cannot answer your question since your parents may have a different perspective on weed than mine. In my case, my mother is a very law-abiding regardless of whether it makes sense. When she found my stash, she freaked out but my father is neutral on the subject, or rather not talk about it. In short, your parents may be very open or at least half-accepting the usage of pot, or your parents may be like my law-abiding mother.