Really? 'Cause I was thinking they were too formal. We didn't have a graduation, but they wanted us less formal at the dance. Definitely check with them. Also, is modesty an issue? While they're all beautiful, some look more like lingerie than dresses. I mean, it's an eighth grade dance. They might want you in something that at least covers your shoulders (or at least have straps that aren't spaghetti straps), and there might be a length requirement.
At the dance I went to, girls were being asked to put sweatshirts over their dresses if the administration didn't like them. (And by didn't like them, I mean, thought they were 'inappropriate', not 'that is one ugly dress!')