Licensing is weird especially in schools. It may very well be practically free for them to license. Or for very small numbers of computers they might be able to come out ahead by only needing to hire tech staff that are competent with Windows compared to the cost of staff competent with Linux. Put another way, in my IT degree program every single person in my graduating class was very competent as a Windows admin, but only a handful of us were any good with Linux (with a couple actively avoiding Linux for being different)