Anything is possible. I never thought that yearbook portrait was from her senior year because she's not dressed appropriately. Didn't most girls dress alike for their senior pictures? Guys would be in a suit and tie but girls would wear the same feather boa or wrap and the identical string of pearls or everyone would be in a graduation gown without the cap (hey, it's an American thing).
Even if there happened to be a photo of her in the 1960 yearbook, it doesn’t mean that she graduated with the class. Senior photos are usually taken in the prior summer or during the first month of classes. The book is usually ready for publishing several months before graduation.