I actually used weed as a study aid all through university, and smoked in high school. I only know a few other people who did this though. I would study for 2-3 hours, take a break and smoke, play a game of disc golf or hang out for an hour or two, and then study for 2-3 more hours. It turns me into a studying machine. I graduated with a 3.84 in Zoology Bio-Medicine, and am now studying medicine. I don't smoke anymore, but I don't notice a significant increase in how well I do in school. Take that as you will, but I don't recommend it unless you are disciplined and responsible.
Oh yea, I forgot to say I did pretty bad in high school, but that is because I never did anything outside of school, and skipped quite frequently.