To answer "Who decides when someone is a slut?" in most areas it's the old women playing cards and eating lunch (excuse me, taking lunch) at the Senior Center. They'll usually cut one of their own out of the herd for the designation and then move on to the general population.
Since this thread is almost ten years old (actually seven) I'll just say that school administrators have been editing yearbooks, both pictures and text, for decades. Various court cases have established that as the publisher of the yearbook they have that right and it's pretty unassailable.