I went to Catholic school. One day, when I was in Grade 10, there was a girl sitting in front of me. Her kilt was so short that every time she leaned forward in her chair, I saw her entire ass.
So, if you're going to wear a thong under a dress, you might consider ensuring that it's long enough to cover your ass. Unless you want to moon an entire room full of people - in which case, go right ahead with your bad self.