I would say that any substance can lead a person astray from education/intelligence.
In my experience, people who get "dummed down" by drugs are people who aren't that intelligent to begin with...and they let themselves get carried away by said drugs.
In my life, marijuana use has opened the doors for me to meet many new, interesting people. I have learned a lot about the world just from buying it, and dealing with different people. It has made me more "street smart"...But it has also made me more "book smart"...Back when i was in high school and started smoking weed, i used to sit in my room and get high and watch history channel, discovery channel, etc...and Just sit on my comp and surf the net...Gathering intelligence along the way.
For me, getting high heightens my senses and makes me more aware of things. It also made me more able to retain information and "pull it out of my mind" more easily...
Case in point: I got high before taking tests in Science/Algebra/Geometry etc...and i would always get a B+ or better.
I also took some College placement tests...and got stoned before each one...I got over an A on each one, and i even got a 100% on my Mathematics placement test whilst stoned.
So, in my experience...smoking weed has made me a more intelligent human being whose' more aware of his surroundings.