Maybe this is too reductionist but I once heard someone describe it as, "there's nothing fundamentally dangerous about pot, but one of its (often intended) effects is that is slows our engines down. And some of us need to be firing on all cylinders." School was a tricky time for me. I'm thriving intellectually now, but I was a C- student pretty much 4th-12th grade. I needed those cycles that pot would have robbed me…
If you use it right you don't just vegitate. It makes you far better at many tasks and can be used to do epic amounts of work so long as you get your work that requires a lot of thinking done first.
The problem is confining to a narrow usage pattern. It is just impossible for 99.99% of people over 30 years of their lives. All will make mistakes at one point.