I don't know how it is in other countries but I don't think that someone can force the teachers to use windows.
However, government agencies usually get discounts on proprietary product and actively advertise them to each and every school. In Italy, they went as far as granting money to music school _only_ if they buy a specific kind of computer and a specific release of OS (we've been told this by an angry music teacher, but nobody managed to go deep in the issue, as far as I know, after we got the official number of the directive).
If a school switches a computer lab to linux, so what? If they use linux instead of windows for wordprocessing or editing source, who cares?
I am not sure about this, but official teaching programs may happen to be bound to specific sw products. At least, when I was asked to teach in a social structure, I had to refuse teaching informatics because of the programs (fortunately maths and physics were still not bound to software at the time).
/alessandro