A couple of suggestions:
First, see that your clothes fit well and are in good repair. Get those waistbands adjusted to fit, and have superfluous material on your shirts taken in. Trim loose threads and repair or toss damaged clothes (unless that's an intended part of your look).
Second, begin to consider a broadly-categorized personal style that can be expressed with popular casual clothing, including some of the clothing you already own. (If you have a keen-eyed friend willing to give feedback, great. If that friend is a well-dressed girl, even better.) What is most expressive of your character and most comfortable for you? What will allow you to be MORE of who you really are? Muted or intense; classic monotone or colourful; funky or approachable? You don't have to go overboard -- at all -- in expressing a personal style; it's just a subtle way of bringing your attire together, to make the elements of your wardrobe look like they make sense together.
In my experience, 'casual but well-groomed' seems to be most inviting, or shall we say least off-putting, to women.
Finally, if you're upping the ante and looking to go more fashionable or formal in any facets of your wardrobe, there are two areas that have a high impact without making you look dorky or unapproachable: footwear and outerwear.
Stylish (not garish) shoes and a nice jacket of your choice (whether a bomber, a trench coat, or a sport coat) will give you good presence on the radar of the opposite sex. It's well known: many (most?) women check out shoes. The rest of your clothing tells a story; but the shoes, to many women, tell more.
Outerwear is the first thing others will see when you're outside and when you enter the classroom; and the fact that you can remove it makes an expensive/stylish/funky piece less threatening. Just make sure it doesn't clash with what you're wearing underneath. Support the story you're telling with the rest of what you're wearing. It gives the impression that while you have an eye for style, you're not attached to it -- you can take it off, kick back, and be comfortable in your clothes (and by extension, be comfortable in your body, which has always been the crucial element of attraction).