I know pot-heads who have attained prestigious academic distinctions and a guy who smoked LOADS who became a partner in one of the top law firms in San Fran while still in his twenties. I see guys I knew in High school who can barely keep their job pumping gas or whatever and are obviously smoked out, with the tight voice and everything.
I have read quite a bit about the effect on the brain, but don't really care enough about the issue to regurgitate it. Suffice it to say you can never underestimate the confounding effect of individual physiology, and the effects of THC are cumulative. Think of it like steroids. You go on for a while, it takes a while for the HPTA to rebound. The longer you're on, the longer the rebound (I know this is not true for everyone, individuality again). Pot has a similar effect for a heavy smoker, substitute dopamine for testosterone (to name just one), substitute the hippocampus and the hypothalamus for the testicles (kind of). Occasional use within a timeframe the body can metabolize the THC will not suppress brain function, but if you take a chronic pot smoker and deprive him of pot, he will suffer a neuro-chemical crash. You make him do a test then, and he will score worse than if he were totally fried.
I knew a guy who would give it up before exams. The first time he did it, he only gave himself a month, he said he did crappily because he was totally crashing. He have himself 3 months after that and loaded up with Vasopressin and Deprenyl and all kinds of shit and was in tip-top form. He was a very heavy smoker the rest of the year though. A real wake-and-bake man.