Get iWork!
I've owned both MS Office for Mac 2008 and iWork (NOT iWORKS!) and iWork is infinitely times better for the average student. The interface is very simple and intuitive. The programs bundled in iWork barely use up any resources and open documents very quickly.
MS Office for Mac 2008 is the complete opposite. Its interface is a cluttered and laggy, it takes a long time to open documents, and it drinks ram like orange juice. Not to mention it takes days for any of the programs to open.
Yes, you may encounter some incompatibility issues, but iWork can export files in Microsoft's formats and I haven't had any problems so far.
It takes some time to get used to, but iWork becomes a MUCH better and useful program over time. You will be in awe of how simple and effective iWork will make creating beautiful documents. I have switched from Office to iWork and I will never go back. It might also help to know that iWork costs half as much and Keynote absolutely DESTROYS PowerPoint.
Apple makes everything so simple; and now the've added that to schoolwork