Define "Semi" Formal, if you please?

So today, I went "semi-shopping" with some girlfriends. Now, by "semi-shopping" I don't mean that we were only halfway perusing the countless racks of clothing at our local mall - that feat isn't even possible for such hardcore consumers. Just kidding. No, I mean that we went shopping for this odd concept called the "semi-formal" dance - that awkward social occasion in high school where no one really knows the dress code. I, as a lowly, unenlightened freshman, was especially confused by the lack of specifics in the name; I mean, really, semi-formal? So, I can just show up in jeans, a t-shirt, and wear some kind of crazy hat/bow tie mix, and call it fashionably correct?? No, this is not true, I've learned. When I showed up in Dillards, fashionably late (no, okay, I was just late. Car trouble. It was sad.) I came upon these racks...and racks.. and racks... of gorgeous satin and silk and polyester-blend confections of short dresses, amidst which I found my friends. I was secretly blown away! So THAT'S what semi-formal means, I thought! Wear the equivalent of a beautiful PROM dress, but several inches shorter. And so I became enlightened. My friends all ended up choosing awesome dresses and some really intense heels; I was super happy to be able to help pick them out. But, I'm still at a loss; and the thought of showing up in a t-shirt and jeans is still pretty hilarious. What to do, what to do?