Schools
Because 99% of the curriculum-conforming software out there is Windows. And precisely 1% of the stuff on Linux etc. that *claims* to be "for kids" isn't curriculum-conforming (and is therefore a waste of time except for "spare time" activities - i.e. when a kid has finished their work). Look at 2Simple Software that produce tons of Windows programs - I could write any one in an afternoon with a graphic designer, but they are focused to certain parts of the curriculum, updated regularly, and are simple to learn. Shame they are Windows-only.
I've worked in school IT since I left uni. The hardware and OS are basically completely the same. What matters is the end-user application software. There are precisely zero big-name educational suppliers selling Linux educational software, even when it's been written in a cross-platform library and allows them to pump out Windows and Mac versions from the same codebase.
The stuff that *does* run is mostly generic applications (so OpenOffice *can* and *does* replace Office in many schools, and TuxPaint is incredibly popular in primary schools - because it effectively replaces the next-nearest equivalent which is RM Colour Magic) and that's sufficient for Internet research, writing notes and printing and some very basic tasks - they are the pens and pencils of the IT world, but what about the textboooks? IT is in *every* subject and there is a big fuss about linking into the curriculum content and there is nothing on Linux that even *tries* to do that. A brilliant piece of software with a million features is useless if it cost £100 and only satisfies one line of the curriculum. Instead a crappy, years-old, Quicktime-based point-and-click game designed for Windows 95 that DOES satisfy the curriculum effectively is worth a lot more, and a lot more investment in IT to get it to run.
Some pilot schools are entirely Linux. This is usually done by, for example, buying a subscription to educational online content that has UK curriculum focus in all the subject areas (companies like Espresso Education specialise in selling schools Linux boxes that have Apache, Squid and 500Gb hard drives that download educational content overnight so the whole school can play a curriculum-aimed video / clip / game simultaneously the next morning - updated every day with every single change and news-relevant topics). Most schools hate that because it locks you into a particular service that you have to pay for every month and when you stop paying, you lose everything. So they stick with crappy, years-old, supported programs that they've owned for years, that are curriculum-oriented, that the teachers are familiar with and that run on anything Windows (with some tweaking).
Software as a service is pushing into schools at the moment (hell, you're basically renting Windows now too, so why not either rent a whole system or move away and rent an online service?) and that's making educational software a bit worthless. But the largest manufacturer of interactive whiteboards (SmartBoard) has had Linux drivers and application software for years, and the hardware is all Linux-compatible (I push Linux into every school I can, where it suits their needs), and the infrastructure is Linux-compatible (you can logon to Windows Servers with Likewise Open quite easily from Linux), but the end-user application software isn't. It doesn't even exist. What does exist is one or two guy's ideas of what everyone should teach (and that's the worst thing you can assume of another teacher) and doesn't cover even the most basic of needs.
When all schools are on Software as a Service, then 99% of the thin-clients, servers, etc. they use to do that will be Linux, because it really doesn't matter any more. But the fact is that the applications ONLY exist on Windows, at the moment, despite there being a Linux market. You should have gone to BETT in Kensington Olympia last week - there's not a mention of anything non-Windows in their software section and what looks like rubbish to you and me is worth THOUSANDS to the teachers because it covers their portion of the curriculum completely and is up-to-date.