I'm a Mac bigot at home but I try to speak fairly about the Mac/PC tradeoffs when we have discussions at school.
Our school has more Macs than PCs because they make sense for us. I try to give teachers and parents fair assessments when we purchase one platform or the other.
In the long run, I find Macs easier to set up, easier to train a beginner (teacher or student) to use, and easier to maintain through O.S. upgrades. All-in-one models (from Performas through iMacs and eMacs) suit schools - the less components and cables the better. Apple promotes and packages software that suits schools. Apple provides online resources for teachers to plan and share lessons.
Does this make Macs a shoo-in? No. PC hardware is usually a less expensive initial purchase. There are more total software titles, even though there are plenty of quality Mac titles.
People don't often mention this, but at a small school, a lot depends on having school staff or volunteers familiar with the platform you choose. There are typically more students, parents, and teachers who are familiar with PCs. If I hadn't been there as a volunteer, they probably would not have leaned so heavily toward Macs at this school.
And having the best equipment and software doesn't matter at all unless the teachers know what to do with it. That's why a teacher who is already familiar with Windows or Windows-specific programs is often better off with a PC.
Having a mixture of platforms is also a tradeoff, compared with mandating a single platform. It is more work to maintain a mixture of computers on campus. But I think students should use both Mac OS and Windows. If they can use Unix too, so much the better. I disagree with those who say "Students should use Windows because they will need it in their jobs." I argue that any O.S. being used in middle school today won't be used in business when these students enter the job market. They need variety and the ability to learn something new, not job training on Windows XP.