I smoke weed about once a week during the school year (much more often during the summer) and I'm one of the top students in my class (this is a private school too). I have a 4.0 while taking all honors/AP classes. In fact, by the time I graduate, I'll have taken nine AP tests, four of which I have already passed (top score on two.) I say this not in a bragging way but to show that people who smoke weed can do very well for themselves. A lot of kids from my school smoke occasionally (it's more of a drinking school, really), yet that doesn't stop us from being one of the top private schools in the country.
It's all about balancing work and fun. I've been smoking nearly every day this summer because I want to let loose before entering another highly challenging schoolyear. Weed really motivates me, both in the short term (I work really hard during the week so that I can reward myself on the weekend) and in the long run (I want to go to Princeton and make a shitload of money so I can buy all the weed I want). You might find that it's really not that hard to keep a mild habit instead of a serious one if you have something occupying the majority of your time (in my case, homework and extracurricular stuff). This way, I can accomplish what I set my sights on, have a pretty low tolerance, and retain sort of a balanced lifestyle all while still enjoying weed.
Oh and if you get really good grades you can work wonders against anti-drug bitches, because you have concrete proof that a pothead is smarter and more successful than they are. You can both fight against stereotypes and avoid suspicion (no one expects an overachiever to be a stoner)