Either of those will work fine. OpenOffice is of course free, and it can save files as .Doc, so they will open with any Microsoft Word version.
However, Office for Mac is your best option if you want things to be absolutely like Office for Windows. It is also worth looking into for school since you will need Microsoft compatibility for everything you turn in. Apple has a Student version here:
http://store.apple.com/1-800-MY-APPL...nplm=T9188LL/A but I would look into buying it at school. It will be much cheaper.