Usually middle school dances are casual. They're also hot and sweaty, so wear your favorite pair of shorts, cute top, and wear extra deoderant. If you live in a cooler area, wear a nice pair of skinny jeans.
If its a semi-formal and ^ up, you should wear a dress, or a nice skirt and shirt.
Also, wear shoes you can dance in. Don't wear heels to a causual dance, and if ballet flats give you blisters, opt for your sneakers instead.
Last, add a coat of mascara and gloss, and spritz a lil perfume. You can also do your nails and hair, but its not necessary. Because its so dark, you cant see your nails, and because its hot and your moving, your hair will most likely get messed up.