You work almost exclusively with the common ground, the common conventions. Teach them file managers, "programs" in a very general sense, windows (being able to resize and position them, the window buttons (to close, maximize, etc.), window menus, etc.), the concept of saving a file from a program, file extensions. You take little pauses to explain how things may differ slightly between OSes.
You take the same approach with programs (web browsers as opposed to simply Chrome, spreadsheets as opposed to simply Excel, etc.).
Once they have the common ground, you can expand on differences, but always OS- and program-neutral, because schools shouldn't be a marketing tool.
The idea is that students should be able to use any computer in a very basic extent. It doesn't take much.
> (Though, how do you choose which distro to use for Linux? Or do you just make them walk uphill each way on the command line?)
You pick the one that has the most common ground with the other OSes.