I think the key is people, not hardware or software.
1. Running labs requires technical skill for both setup and ongoing maintenance. The trouble and expense to maintain a support staff for both platforms on an ongoing basis could outweigh the benefits of giving students exposure to both platforms. You'd save time and trouble to establish one platform and stick to it. That could be either Macs or PCs.
2. I think the emphasis for computers in schools should be on the teaching that uses them, whether it's Internet research, building up writing skills using word-processing, using computer-based tools to learn science and math, and so on. Software for doing this is available on both platforms. Students adapt easily and will probably learn as much on either platform.
What ages or range of grades will your school cover? If they are older, it would be a bit nicer to offer them access to a variety of computer experiences, but even if they are high school age, computers will have advanced past what's available today by the time they finish college, so I wouldn't consider it especially important to mix your platforms simply to offer them more exposure to the computers currently on the market.
I maintain a Mac-based computer classroom for a middle school, grades 6 to 8. We chose Macs because the teacher came from a Mac background (a former graphics professional) and because as the support person I could help the most with Macs. The software we use is Adobe's suite, plus web browsers, and lately we've been using M.I.T.'s Scratch application; all of these are cross-platform, so the decision was based on people, not platform tradeoffs.
They say that total cost of ownership is about the same for Macs and PCs. It seems to be basically true; while the Mac purchase price may be higher, they come with more included software, are easier to maintain, and they tend to last longer since they are well made and come from a single manufacturer.
We benefit from the ease of maintaining our Mac setup, with a Mac Pro running Mac OS X Server and iMacs for students. We keep home directories on the server, switched from netbooting to local booting this year (that's another tradeoff), and use Apple Remote Desktop for much of the software maintenance. We're glad we don't have to deal with malware issues regularly on our Macs (we do on the individual PCs in other classrooms).
On the flipside, we suffer from less-good support from our school district, which puts most of its effort into Windows. Also, having to spend more up-front to set up a lab, and less for ongoing maintenance, is actually harder to fit into a steady-state school budget.
Occasionally, we run into Windows-only software and haven't gotten the chance to use it, but if we went to the trouble we know we could use Windows under Bootcamp on our Macs.
There's also the practical consideration that you have to "sell" your private school to prospective parents. Macs tend to look modern and have a wow factor, and that makes a great first impression. But we have parents who argue that kids need to use Windows because it's better job training, or that kids should use computers that match what they have at home, meaning we should give the edge to PCs. We can and do argue these cases on the merits, but it's a constant battle to defend the use of Macs to people with a negative bias.
