Macs are the main competition to Windows, not Linux.
While this may be true, I would like to think that schools who "go down the Mac road" would be held accountable for that decision by those who finance them. I can't conceive of any justification for purchasing significantly over-priced hardware (in comparison to the relatively cheaper and virtually identical vanilla PC hardware).
Whether you like it or not, Microsoft pretty well dominates the commercial sector. Training people to use Mac products, whilst arguably an improvement over a Microsoft-lock-in, isn't going to be either the most appropriate use of funding and in one fell swoop limits the sheer quantity of available software to run on the chosen OS (although that's arguably a good point!)
The issue of cost (or more correctly value-for-money) leaves only one clear candidate, and that's Linux. As with Mac, the choice of software is limited - whilst Linux has (far too big) a selection of software available, I'd argue that the quality of much of the software (outside of the "key" mainstream apps) is somewhat limited: That said, Linux has more than enough (as does Mac) high-quality, feature-rich mainstream apps (such as Firefox for web browsing, a port of Adobe Flash, a plethora of Multimedia apps, OpenOffice, The GIMP etc.) but delivered in a value-for-money fashion (i.e. most of this software is free and well-supported by the community).
I'm not grumbling per se - I use Linux myself (Linux Mint if you must know), and am an advocate for Linux where possible. However, although Linux has matured over the years, it is still not quite (imho) "ready for the desktop". There are a number of design fragmentation issues that need to be resolved (choice of desktop manager, multimedia software, configuration toolset etc.) - most end users simply don't care what desktop manager or MP3 player they are using, as long as that software is stable and of good-quality both from a performance and appearance perspective, simple-to-use (i.e. intuitive), compatible with most (if not all) other software (in the case of desktop managers, for instance), and of sufficient caliber to insulate them from the intricacies of the underlying OS (eg by including adequate configuration applications, plugins, codecs or whatever).
For me, though, It's good to see each and every instance where Linux is gaining a foothold - not "just" because it's Linux, but because it represents an informed choice of a value-for-money OS, and also because it goes one step further to aiding in the catch-22 of "Linux for the desktop": Increased market penetration will (hopefully) lead to improvements, and improvements will lead to increased market penetration. As a parent in the UK, It would be nice to see Linux gain a significant share in schools here too: As a taxpayer, I am effectively contributing to their funding and take great exception at the amounts of money being spent on proprietary technologies that are, frankly, unnecessary. The money would be greatly appreciated and of use in other areas, and Linux is a credible solution to liberate these funds.
A right is not what someone gives you; it's what no one can take from you. -- Ramsey Clark