i'd just grab the felonly amount of weed in your state (dirt weed, of course), toss it in her backpack/desk/purse/pillow/etc, and then go tell the cops about it (having moved all your shit into a trusted friend's custody, of course). then, while you visit her in jail, say very softly, "now you know not to steal my shit anymore, don't you?" then calmly get up and walk away.

or just tell her that you know she's been stealing her shit, then punch her in the ovary and cut her nipples off with crayola safety scissors.