How can I cover my back for Prom?

You can wear a racerback dress which would cover up your back a little, or a short sleeve dress. But to be honest you should wear whatever you want and not worry about it. I have a ton of friends who have really bad back acne but they just go with it and no one looks at their back anyways. Don't miss out on prom just because of it. Trust me, you'll regret it.