You should bring slices of bacon to school and place them on her desk or on her locker. And once she starts noticing it, get more and more subtle and start placing the bacon on her route to classes and stuff.  It might be funnier to cook the bacon and bring it there, because if you bring an intact piece of cooked bacon she's really going to wonder wtf is going on. And she might be forced to realize the lengths someone went to in order to subtly call her fat.