I've been sorta keeping up with this thread from a distance when I've been able to be here. As a former teacher--and as someone who enjoys debating with intelligent students, friends and arguers here and in other places--I encourage you to take the high road. Let the teacher present the information he's going to present. Then respond by saying that there are lots of very strong opposing viewpoints about the harmfulness and toxicity of cannabis and lots of solid evidence upon which those are built. Explain that there is lots of information about the benefits of cannabis and probably far more benefits that we haven't yet discovered since testing hasn't been allowed with it here. Present your argument in a way that makes it positive and credible, citing your sources. A variety of sources from books, Web sites, TV shows like the History Channel's "Illicit" series will be much more credible than just a single source. That way, although you may not ever change the teacher's viewpoint, you'll have left an impression in the back of his mind that you might have had some solid evidence upon which to base your argument. You'll also have planted the seeds of fair-and-balanced argumentative information in the minds of your fellow students, which may be what counts for more in the long run than anything else.

You can get very far in education by being respectful and having good sources. Let's hope he's a fairly secure teacher who can handle being questioned and/or debated. If he's not--and they're out there--this is why it'll come in handy to still be respectful and deferential: so that you don't doom yourself to a less-than-fair-treatment for the remainder of the time you have to endure him.