As someone who is currently in a school that uses only free and/or open source software I have to say that this is a terrible idea. 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…
also, it doesn't save the school any money anyway since they have to hire Network Admins and other IT people with Linux experience
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.