Wow, you have a ton to choose from.

http://www.craigslist.org/pen/bik/74947230.html
jamis road bike for $600

http://www.craigslist.org/sfc/bik/74941035.html
motobecane $150

if short
http://www.craigslist.org/sfc/bik/74941035.html
carema $500

cannondale $250 ??
http://www.craigslist.org/eby/bik/74881953.html

miyata $400
http://www.craigslist.org/pen/bik/74879927.html

trek 1000 $350
http://www.craigslist.org/sfc/bik/74811135.html

diamondback commuter $75
http://www.craigslist.org/nby/bik/74805733.html

jamis cross country $200
http://www.craigslist.org/eby/bik/74793911.html

How to decide?
1. you want a road bike, not a mountain bike. It's lighter and easier for commuting.
2. Can you safely store your bike at school? if so, pick the bike you want, if not pick a cheap one.
3. go to a couple bike stores and get sized so you know what size bike you need.

If it were me,
unsafe school-- diamondback
general all around-- jamis cross country, or trek 1000, or cannondale. Depending on which fits you best and has the easiest shifting
if you can afford it-- jamis road bike

stick a finger in the air, put down your money and punt: the jamis cross country.

You have a ton of bikes available, it's end of the school year. if one is gone, others should appear on the market. Have FUN.