Downtubes are very solid bikes with good value. Great for someone just getting into folders. The parts used on Downtubes are very standard and widely available, so I'm not reluctant to recommend them. Dahons are good brand also.

Which area of the City are you riding along? Are there hills on the route? That will determine what type of bike you should look at. You will also want to consider the weight of the bike if you have to carry it up stairs or into the classroom (unless you have big biceps ).

Have you tried Warm Planet Bikes at the CalTrain station? They have a variety of folding bikes. Also, there are many Dahons on SF Craigslist right now for good prices.

I live in Half Moon Bay and own several folding bikes. You are welcome to test ride the ones in my collection to try different styles.