I don't necessarily disagree re: OpenOffice/LibreOffice/whateverit'scalledtoday. It is pretty good in saving most basic stuff in vanilla .doc although I wouldn't call it perfect. I've never used the spreadsheet and presentation apps, so I can't speak to those but let's assume that they are also good enough for the majority of school work.
The problem is that this isn't the only suite of apps that kids have to use for school work, your sisters' experience notwithstanding. Some of those apps, luckily, are done in Java, so they can work on Linux systems, but not all. A friend of mine had to deal with a number of Windows-only apps that are required for his kids schoolwork, and he eventually gave up and installed XP, rather than deal with Wine or contacting the school and the publisher. So while in your particular case and context, Linux works fine, you should be careful about extrapolating that, therefore, Linux works fine for everyone in the same context.
As for your contention that most stuff is done through a browser, online, it's true that more and more stuff is now done through browsers. That's not as great as you imagine. Do you know how many educational applications have browser version checking and how many of them actually "require" the use of IE? The reason, of course, is that they want to minimize their testing & support, and the webapp might work perfectly on your browser of choice once you tweak the browser agent string. And while you may be able to easily figure that out and do the necessary tweaks, this isn't about you. It's about a kid that has no idea about Linux, Windows and browser agent strings, or about their technically inept parents.