Look for something used on craigslist, sdreader, etc... If you're going to ride it at school, it needs to be beyond fugly so that you still have it after the 1st day.
You'll get much more bike for your $400 by going used. Something $400 new is going to have low end components. It will also look new which will attract thieves. Oh, there's a bike swap meet at the SD velodrome this weekend not sure if it's sat or sun, you can always get a deal on something decent there. Good luck, and get one that's sized right.