Anything you do to try to get him into trouble will just come back to bite you. You have told both your parents and a teacher, and they either haven't believed you or have told you to lay off. That was actually good advice. If his drug use were putting the school in jeopardy, people would care more, but you aren't saying that he is stealing school funds for his pot habit, or that he is making bad decisions because he smokes pot, just that he is doing it. If he is really doing anything as blatant as smoking pot in school, he will get caught by someone with more credibility than you have. What you are doing is trying for vengeance, and it is not only mean and childish, but it will harm you at least as much as it harms him.
I should add that when I was a camp counselor years ago, I took a group of kids horseback riding along with one of the camp directors. We stopped at his cabin to pick something up, and I saw a roach (marijuana, not the insect) in his ashtray. He knew I saw it, and he treated me abusively the rest of the summer in an attempt to destroy any credibility I may have had if I reported it.