I had a friend in college that did literally this. Guy sold weed throughout his time there. While he was a senior, his supplier who had graduated a year earlier hooked him up with an internship at his company's trading desk. Went to work on Wall St after graduation.
I was always amused that the marijuana business was more important to his career after school than his studies (I think he was an environmental systems major). He's a commodities trader now, which is fitting.