Students really don't get this feature at all, as you never truly know what it's going to do. One student showed me that his green button (with one app) didn't do anything at all. Another time, the green button caused the window to slide partially off the screen, concealing some of the information he needed. Other times, it went full screen and freaked them out, thinking they did something "horrible" to their mac -- it somehow "deleted" their dock and menu bar!! Help! How do I get them back? ;-)
Misunderstandings like these happen all the time. Systems all have their own logics and quirks, and misunderstandings are not in themselves reasons why Apple should favour one over the other or offer options. I happen to know some people who don't understand the idea of dragging applications into the Applications folder for installation and who become confused when applications refuse to start because of that; ultimately preferring a package installer, like on Windows. I also know people who don't understand that closing a window is not equal to closing an application and who prefer a force-close approach with helper tools that continue to run in the background, like on Windows. It's perfectly reasonable to think that way, but it's also perfectly reasonable for developers to stick to the Mac way, ultimately for reasons that go beyond just user preference.
The green button is only unpredictable in the sense that it doesn't expand the window, but adjusts it. People who say that this feature is unreliable often don't understand what it is supposed to do and don't engage with it properly. Normally there is no reason to use it, unless you find that the current window size is unacceptable and you need something to quickly readjust the size to something that allows you to view the content better, either a bigger or smaller window. I accept that the implementation of the green button is not ideal and that some developers have implemented it poorly into their apps. But that doesn't mean that it's a poor feature in itself. Apple's solution to this confusion has been full-screen mode, where developers have little control over it. This change of default behaviour now lies at the heart of this debate and I think that your students in particular should appreciate this.