I graduated summa cum laude from my masters while vaping regularly and am now about to finish my phd. That being said, cannabis can absolutely hurt your grades if you're not careful with how you use it. In my case, I treated my studies like a full time job, worked part time on the side, and vaped small amounts (around or less than 1/10 of a gram) in the evening to relax, around 6 days a week. However, I don't personally know any other cannabis user that was able to maintain such a balance. I've also now limited my use to the weekends, because it does hold your mind back a little bit even if you're super careful and work your ass off 8+ hours a day.