While I agree with most of your points....I gotta take issue with this one. I saw it in college all the time - kids that were "on" marijuana more than they were off of it. They didn't last long in school. I'd say that among users, it was about 50-50 - kids that didn't know when to stop, and kids that only smoked occasionally.
I always thought it was a real waste. Lots of smart teenagers got into dope and eventually failed out of school.
To be honest, I think I probably saw it more with alcohol, though. Hard to pass your classes when you're out partying four nights a week. I know that much from experience - learned after failing a semester to limit my partying to vacations and weekends.
And, in a related point, My Dad went to college in the early 60s. He says he didn't even know anyone who had tried pot. I went to college in the early 90s. While I was in school, I only knew of 3 people (including myself) that hadn't used it. What a difference a generation makes.
I went to school in the '70's on the East coast. The majority of people I knew used weed. A few used too much, classified as blowing at least one semester of school. I damn near did, though, although I wised up. The only people I knew who actually flunked out, though, did it while abusing alcohol.