I smoke pot constantly. I really should cut down, because inhaling smoke is never good for one’s health, but for now, well, it’s pretty much 4:20, 24/7.
When I first started smoking in high school, I watched my grades plummet, though that may also have had to do with the fact that I worked until 11 PM every weeknight and could never go to sleep until I’d watched the back-to-back episodes of Wings on USA after returning from work.
Now I go to a technical college for computer networking. If they offered a similar course at any local university, I would have jumped on it, but they don’t. Funny, too, because there’s more than enough to know about networking to fill up four years and then some. Since I started school two years ago, I’ve smoked all the pot I could get my hands on. The weed here is mostly Mexican crap, so I usually end up smoking one to two ounces a week. I know, I know. That’s a lot. You don’t have to tell me. I’m the one who has to clean the bong every freakin’ day.
Anyway, since starting school, I’ve gotten all A’s except for one B (idiot teacher that gave everyone B’s for no apparent reason) in my computer classes. For my other classes, my grades depended on how much interest I had in the class, a crappy habit that I’ve carried since long before I started smoking pot. Still, even with that lack of interest, my English teacher called me her most well-rounded student after having a few conversations with me about literature and other subjects.
My computer teachers routinely come to me with questions they can’t answer. Often, they ask me to explain difficult concepts to the rest of the class. I’m considered a total nerd by just about everyone. Few of the students and none of the teachers know I smoke pot, even when I’m sitting in their classes mere minutes after smoking. It almost feels like I’m living a double-life sometimes. 
I also entered a state competition for networking and got a silver medal. Yes, I was stoned during the competition.
Wow, that’s a lot of horn tooting. Sorry about that, but I guess I at least provided another affirmative answer to the OP’s question. Yes, stoned people can do just fine in school. Not every stoned person to be sure, but many of us.