Hi Emily,
It's good to remember this general idea... Macs and mac software is designed to read, import, or otherwise handle documents created by Microsoft software. But it never goes the other way - Microsoft products won't read mac stuff. An exception is with MS Office 2008 for Mac, the interface with MS Office for PCs is easier there. With iWork software like Pages, it's up to the person on the Mac side to put things in a format Microsoft software can read. Luckily, mac makes that easy.
If you want to open a pages document on a PC with MS Office, you need to save your Pages document in Word format, either using File>save as>save copy as Word doc OR Share>Export>Word. Then, at school open the Word document. If you make changes, just save your document and when you get back on your Mac, Pages will open the Word document (file>open and make sure to choose the .doc file). Save that as a Pages document. You can go back and forth like that easily, just always remember: mac reads microsoft, microsoft doesn't read mac!!! If this is too cumbersome, I suggest buying MS Office 2008 for Mac. There is decent educational pricing on it, and it is a nice product. It will seem familiar if you are used to Word 2007. It's just more expensive and less stable than iWork.
If you want to open all your Word documents automatically in Pages, just follow the instructions given by Peggy earlier in this thread.
I highly recommend the Genius Bar at the Apple store since you are new to Mac. Or sign up for a one on one ($ I think) or free class in the store. It's worth it. But I am confident you will love it. You will probably get to hate the PCs at school so much you'll buy a laptop. 🙂 I refuse to work on a PC now.