Honestly? Wear what makes you comfortable. You’re there to learn, not a fashion show.
But I think that it really depends on the person and the day. Most people tend to wear just shorts/jeans and t-shirts, or sweats. Sometimes, you’ll be jumping out of bed and going to class within twenty minutes. And if you’re not from Missouri, remember that it can get cold quickly (as early as October) so keep in mind that warm clothes ARE necessary.