Your first co-ed dance as a preteen can be terrifying. Between raging hormones, expectations created by decades of school comedies on TV and the big screen, and navigating the weird cultural assumptions of your local community, it's no wonder kids are nervous.
It also makes sense for grown-ups to want to help the kids know what to expect. I mean, I definitely appreciate being told explicitly what the dress code is so that I don't end up over- or under-dressed for the event.
Expectations and ground rules make sense.