Started from the beginning. I'm dense and I need to ask a question.
Kouru's not actually smoking weed, right? Anne just thinks she does because she's a sheltered "good girl" who believes the reefer is a common source of all evil.
Correct, neither Kouru nor her friends use weed or any other form of drugs.
I can't recall exactly where I heard it, but from what I have heard, marijuana usage is not as common in Japan as in the West, or it at least isn't a publicly discussed thing. Many people in the West have some general understanding of what weed does, how it's used, etc, even if they themselves don't use it. On the other hand, the majority of Japanese people don't really know what weed does or how it's used beyond "a thing you smoke that has funky effects on your brain".