I'll second Scrivener for getting your writing projects under control. Its full screen mode is perfect for blocking out all distractions and it was designed by someone who is a writer himself.

For word-processing and formatting I would recommend NisusWriter Pro. It may be more than you would need for high school, but as you get into writing larger papers, nothing beats its killer search/find/replace features, and formatting is a lot more intuitive than Word.

Bookends X is also a wonderful, and very mac-savvy program for managing all of your books and for automatically formatting your footnotes and bibliographies.

I'm also partial to my own programs -- Ready-Set-Do! -- a file-oriented approach to getting things done on the mac. And Speak It On My iPod -- that converts text into audio you can listen to on your iPod or iPhone.

If you try Ready-Set-Do!, you'll want to read the "How to Use RSD" file so you can see how it can help you plan your projects. If you assign a due date to a project -- say, a paper due at the end of the semester -- it automatically calculates the deadlines for all of the sub-tasks you'll need to get done to finish it on time; and the tasks turn Yellow -> Orange-> Red as the deadline for each one approaches. I always wanted a program that would not only tell me when something was due but would also tell me when I needed to get everything else done in order to get the entire project done on time. So I wrote a program that finally delivers that for me. It takes some getting used to, but it's a pretty cool setup.

Hope that helps.