How can I teach Python GUI?

It is possible to write GUIs in Python, but we usually recommend covering GUI concepts using other programming languages, like HTML/CSS or JavaScript. 

If you are very keen on GUI with Python, here are some tips:

A teacher we used to work with has some experience with using Kivy for Python app development -- she gave a talk about this at PyCon https://www.youtube.com/watch?v=_1keMKArpOw and another video on her own channel https://www.youtube.com/watch?v=5on8Ybe41tE

The other approach is to use something like PyQt5 -- https://www.riverbankcomputing.com/software/pyqt/intro -- it's a much steeper learning curve but I've seen (and used) a reasonable number of applications that are built on this.

Another option is tkinter, which is built-in to Python and is easier to use:https://docs.python.org/3/library/tk.html

There's a relatively new project, started by a Python developer in Perth, called the Beeware Project which aims to provide a proper cross-platform app development framework for Python: https://pybee.org/