Sure, but then they have to pay the salaries of an IT department to not only do the OS install on thousands of devices, but also provide support when things go wonky from kids doing dumb shit (it's Linux; there will be that one kid who figures out how to gain su privileges and convinces a couple others to rm -f / their shit). The same thought crossed my mind, but these are low spec $200 laptops that I really don't think it would be financially viable to do so.
There's a pretty wide distinction between navigating a linear OS like Windows and OSX, and a flavor of Linux, especially if the teacher isn't familiar with it themselves.
I don't necessarily agree it would be all that difficult, but either way I can't think of a more essential skill to be teaching with those chromebooks.