Its far better to show up to class looking nice, than like a slob. Personally, sandals are a no, sweatpants are a no, and pajama pants a definite no.
I either go with some khaki shorts, and a nice button up, or my gym shoes, and a tshirt with khakis shorts.
Dont overdo it though, and dress like your going to formal, or look like your sponsored by Ralph Lauren haha.