I work in schools, and the quantity of software that cannot be replicated with free/open source is colossal...
Its very easy to say use a Linux based teaching alternative, but the truth is the teachers have enough issues controlling the kids and using Microsoft, let alone having to cope with totally alien software and operating systems.
We trailed the use of OO and found it a pain to deploy consistently, We use OSX on some machines, but they are now gathering dust as the staff are having issues coming to grips with the change (yes, we have tried training, but we cant afford to pay overtime to train the staff with more than the rudimentary skills to operate the machines and getting supply in to train staff during school hours is still expensive and counter productive to the students education.)
Our cheap to run MFC's lack proper Linux support, they even run in a limited capacity on the OSX machines, 90% of our educational software is windows only, we have some cross platform stuff, but non MS tends to be a revision or more behind the windows equivalent. - as a comparison, imagine the ios app store is the current crop of teaching resources for windows... how would you feel if your child's education was then downgraded to use less than the availibity on windows phone marketplace...
Office is not the only software used in school...