When money's tight and your main requirement is class attendance, whatever you've got will probably do. If you're looking for guidance on getting duded up and not looking like, well, a student, an Ask MetaFilter thread has some great advice.
In response to the query of "How to dress like a man", a number of Ask MetaFilter members suggest peeking at the Put This On blog and podcast — as do we. As for their own advice, many can't go far enough to recommend that a man pick up a nice pair of shoes — not your average sneakers, boots or sandals, but actual shoes. There's a link proffered for this guide to matching shoes and jeans, and some personal anecdotes of what works:
I like to subscribe to what I just decided to name the One Stylin' Principle. The basic explanation is you take one item of your clothing and go all-out, but leave everything else normal. So, jeans and a t-shirt but some nice black leather shoes. Or, jeans and sneakers but with a fitted button-down. The idea being, Yes, I'm comfortable with what I wear, but I am going to show you that I do actually possess the shared cultural norms to look good were I to decide to do this all the time.
Handy stuff, especially for someone just getting out of school without a lot of guidance on how to dress. Women's clothing that "looks adult" is a lot more complex and varied, we've heard, but if you know of a good, simplified resource guide, we're all ears in the comments.
How to dress like a man [Ask MetaFilter]