Girl, it honestly depends! I've gone to soo many middle school dances (I'm past it now, but I know where you're at!) and typically they're not that formal. Usually girls like wearing shorts and cute tank/t-shirt because if there's SO many people and it gets pretty hot. If the dance is the daytime, in my opinion, that's definitely too formal. However, at night time, you might wanna discuss it with your girlfriends. I know I always feel more comfortable when my girls are doing the same thing, so why not all plan to wear dresses together?! I would say that overall, a dress is okay as long as it's cute, easy, and casual.
Hope I've helped girllll!