Most people get stuck buying an operating system they did not want because some program they had to have was only on that platform. Desktop publishing is one of those pieces of software that can determine the computer and operating system one purchases. Scribus is an open source desktop publishing application that works on all the major operating systems. It also claims to be lighter on the hardware requirements. Of course, it is less feature rich at this point in its development but that should not be an issue for a school computer lab or school newspaper.
People frequently claim that students need to learn on the systems used in the public. I find that this is often a unwarranted requirement because it's the task and procedures that are important. The menus and shortcuts are always changing even within a single product. Take MS Word, for example. Compare Word 2000 with the new Word 2007. Not much being transferred between those two versions. So, unless you are teaching graphic arts to soon-to-be professionals, make your decision on other factors.