Did Bristol really graduate from High School?

I am comparing these two pictures of them.  Both graduated from Wasilla High School, Bristol 2009, Sadie 2010 but their gowns are different colors.  Did Wasilla change graduation gowns?  I am asking because there are no pics of Bristol at graduation, just the one of her in People Magazine where she is holding Tripp.  The one of Sadie here was taken at graduation.