The other day I chatted to a governor of one of our local secondary schools, which recently upgraded from Windows XP to Windows 7. As you note, the teachers aren't always tech savy and many who primarily used computers at school (on XP until then) struggled with Windows 7. Compatibility with very old Word docs created on the old systems was a problem (solved by using LibreOffice on one of the staffs personal laptops as the IT contractors wouldn't allow it to be installed on the system), and pupils/parents who use OSX at home also sometimes have difficulty with homework assignments when particular software and/or formats are required.
The trouble is we live in a more and more diverse computing society. Windows is no longer the dominant OS, OSX and to a lesser degree Linux (and iOS and Android etc. etc.) are becoming more and more prevalent.
The general solution, is either to a) invest in training for teachers and design curricula to be OS/vendor neutral, or b) standardise (if one must) on an OS/app ecosystem that both the school and parents can deploy and support (which apart from private schools with moneyed parents) means a free OS.
it's a chicken and egg problem. the only way to solve it is to make the first move, otherwise in 10 years we'll be in the exact same situation.
Even for worse... no one in the school had the Root password... oh, the horror.
My high school (2005 IIRC) had guadalinex [1] on all computers, with a 2:1 student to computer ratio and no in house IT. Students basically use Firefox, OpenOffice and some other apps for math and psychics, there's not a lot that can go wrong and nothing is so important that you need a person always on site.
It might not save in the support cost, I agree - but barring a (potentially uncomfortable) transition period, it shouldn't cost more either.
It is my experience (from industry) that a competent Linux admin can support about 10 times as many machines as a competent Windows admin, but only costs 2-3 times as much; However, competent linux admins are much harder to find.
The biggest problem is the teachers, they are not as tech-savvy as the kids and introducing them to a completely new operating system has done nothing but slow them down and cause problems. They are having trouble distributing learning material because they are not familiar with the file formats (and most of the students here are using windows, so compatibility is a problem).
There have also been problems with the personal storage spaces students have on the computer network, people are sometimes unable to log in or unable to access their files. It's been a mess.
I'm all for supporting ubuntu, in fact I've been using an ubuntu variant on my laptop for a few years now without any trouble, but using Ubuntu (or any other OS that isn't Windows or OS X) in a school environment just to cut costs is a recipe for disaster.