a 20" MTB-type frame probably is usable for you on-road, but you won't have enough standover clearance to safely use it offroad. a 5'7 / 29" inseam you should be on a 17.5" MTB frame for trail use. but like I said, probably not a major concern around campus.

jsdavis really threw out some good options, the marin in berkley for $175 is my favorite. some of these will go FAST I bet. my local CL churns through bikes really quick, unless they're overpriced.