Growing up in the 70s and 80s, upon reflection, i would gave been put in the ADHD category. Hyper, class clownish, whatever you want to call it. I was mischievous, but not beyond that.
At around 13, i discovered weed. From that day forward i basically was good to go. Grades were not a problem, so weed just turned my noisy brain down enough to chill and focus. For me, it was perfect. But, there's a limit. If you struggle to get Cs, the addition of marijuana may not help. And if you're getting baked instead of going to class, instead of doing the work, and not really participaring, that's not going to help.
Something happens as you go through high school to college too. It's one thing to get high before a general class, with 80 people, and getting high before a college class in your major, with 20 people. Now, you're paying good money to sit with other people paying good money, and taking their education more seriously. Little by little, i saved my weed use for after classes, after the day and work was done.
Now in my 40s, way out of school, there's a middle ground where a small pinch here and there supports my day, almost as if it were medicinal. I would say it neither helps me focus nor prevents it. It sets my head right to a relaxing place where i want to focus on whatever it is I'm doing, and enjoy it more. It's supportive.