I think that if everyone else gets to wear whatever they want, then she should be able to as well. However, if there are rules in place that says what she can and can't wear for the yearbook picture, then she should abide by those rule.
When I got my senior pics done, one of the rules for the yearbook was No Hats. I understood the rule - you don't want someone covering their face with a hat, or have someone wearing a trashing old baseball cap. My favorite outfit that I had at that time included a really nice, blue, wide-brimmed floppy hat. Very dressy. I had my picture taken in that blue hat, and that's the one I gave out to my friends and family. I had pictures taken in a different outfit for the yearbook photo. I didn't sue the school because I wanted to wear the hat!
That's how I see it, too, KK. It should be EQUAL rights for everyone, no matter what their "differences." We are all human, no matter skin color, religion, or sexual preference. Equal rights also means that we ALL have to play by the same rules, whether in legal terms or school rules, or corporate policy. If the rules say girls have to be in dresses for the yearbook photo, then that is the rule. She shouldn't be an exception just because she is gay. She can certainly wear anything she wants for her senior pictures, but if she wants one in the yearbook then she has to abide by the rules.
At least that's how I see it.