When you send stuff onto the internet for anyone to read, it isn't a private matter anymore. You take your chances on who sees what. Just like this post here.
Schools are getting extremely serious about what students (and faculty) are posting online and on their own time. There is an enormous bullying problem associated with social media. In my own experience, I've seen giant school brawls happening on school grounds because someone said something about someone else on facebook and dragged a bunch of other people into the argument. Maybe what your daughter is posting is harmless but maybe it's not. A few years ago a co-worker was fired because over a weekend, she posted "I'll kill you B*&tch" to another co-worker, who was her friend, as a joke. Someone saw it and reported her at work. Bottom line, she was fired for it on the spot.
As an extreme example, the stabbing case in Wisconsin, where the two 12 year old girls almost killed a classmate, started as a way to "please" a character who existed only on the internet.
Read the student manual to find out what the rules and expectations are in your school district. If they aren't clear, talk to the principal.
As for the MJ. It's still illegal in most states and it's certainly not legal in any state for minors to smoke it.