I’m an Australian, I do not know all of your customs, but one freeper did tell me there is/was an occasion during high school, where all the girls of a grade will/did pose in their high school sweaters, and all hold the same pose as the photograph is taken.
She’s a ‘woman of a certain age’ who maintains she has a similar grade photograph in her own album.
To her eye, there are girls missing on the left of the image, there were less girls in the back row than in the front, SAD is an addition/insertion - and if she (freeper) had turned up for that photograph session without her sweater, she would not have been in the photograph she has in her album.
http://en.wikipedia.org/wiki/Letterman_%28sports%29
http://en.wikipedia.org/wiki/Varsity_letter
They can then buy a "letter jacket" or "letter sweater" to display the letter and their status.
The sweaters have gone out of fashion, but letter jackets are still around. It may be that in the 1950s, girls wore letter sweaters and boys had jackets.
So, Ann posing with her club without a letter sweater may be because she was a manager, or she was on the junior varsity team and hadn't earned a letter, or because as a budding class warrior she eschewed status symbols and refused to buy or wear an "elitist" letter.
I'm not saying for sure the photo isn't shopped, but those are all possible reasons why she isn't dressed like the other girls on the photo. It would be nice to have more context -- like the rest of the photo, the rest of the yearbook page, the captions, etc.