You obviously have never watched an effective teacher teach a class using computers, even if you appear to be in education.
It starts up with instructions about how to start the app read from a crib sheet (or often the teacher or teaching assistant setting the programs up before the class starts), and then continues with using the app, which is probably OS agnostic. What use is detailed Windows knowlege in this type of class?
In the UNIX/Linux world, it is possble to set up specific user accounts that just launch the required application. So the instructions become "At the login prompt, type the name of the application, and watch it launch". No specific user ID's per child, you can lock the ID down so that even if the kids find a way to break out, they can do no damage, and the teachers DO NOT NEED TO KNOW ABOUT THE SPECIFICS OF THE COMPUTER.
Your points about supportabillity only count if your support staff are only trained in Windows. Why should they not learn about Linux. It's not like your average teacher who uses Windows XP Home or Vista Basic at home is likely to be able to support a network of Windows systems without additional training. And unlike your average IT professional, they don't CARE about whether it is Windows or not, just that it works like the manual and procedures say it should. They are TEACHERS for goodness sake!
It is possible to lock Linux down so that it will never change until some deliberate action changes it. Try doing that on XP, or even Vista where some programs or other will require Admin rights, and this is likely to open vectors for system corruption in the classroom. If you are really that concerned, you can effectivly make your Linux PC a thin client, or even both thin and fat, determined by what user you login as.
And as for applications, UNIX/Linux programs work from network shares much better than Windows ones, and have done since Sun said that "The network IS the system" in the 1980's. There are LOTS of people who understand how to write applications for UNIX-like OS's that can pick up all of their code and configs from relative or non-specific paths, making the way that the shares and mounts are accessed less important. This means that the apps DO NOT EVEN HAVE TO BE INSTALLED ON THE PC's. Just mount the share or remote filesystem read-only during system boot, and go.
And the best teaching software is bespoke, or at least written specifically to support the subject. The BBC micro, rest it's silicone chips, had huge amounts of subject led programs available, often written by the very people who used it. When schools started installing IBM compatibles, many, many teachers found that there were too few subject lead programs available, and the PC was too complex and had too few development tools available to allow the teachers themselves to write the simple but specific programs they needed. This may have changed now, but there was a generation of teachers who cut their teeth on BEEBs but felt that the new computers in their school were in-accessible or of limited use for their subjects.
I've seen too much "Computer" teaching ending up being teaching particular packages, often Windows ones. Computers SHOULD be used as a tool to support other subjects, not as an end to themselves, except in ICT classes. And these should teach more about HOW computers work, rather than just how to use them.
I've now left the education field, but I have three children in various levels of education, and nothing much seems to have changed since I was there. I have heard of schools who have embraced Open Systems very sucessfully, and only use Windows for a few packages that there are no representative alternatives available, but in most schools, the Windows momentum is difficult to deflect.