Well, I don't know what the offending posts were, but I would guess that the school has rules set by the administration regarding use of the school's property (their iPad.)
Anything written on a social network is subject to prosecution and can adversely affect your daughter's reputation and safety. Maybe you need to talk with her about being very cautious what she puts in writing, whether it's on twitter, facebook, email, imgur, tumblr, pictures posted, or anywhere else on the internet, and even hand-written notes. Once it's out, it can't be taken back.
As far as marijuana - it is illegal in most states, and definitely illegal for minors...I think you have bigger fish to fry than whether or not she can use a state-owned iPad.
There are rules and consequences for breaking those rules in society. I hope she doesn't get into any trouble that will follow her throughout her life.